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Linear equation in 2 variable class 10

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Linear equation in 2 variable class 10

  1. 1. CHAPTER - 3
  2. 2. PAIR OF LINEAR Equations in two variables
  3. 3. INTRODUCTION
  4. 4.  An equation which can be put in the form ax + by + c = 0 where a, b and c are real numbers {a, b ≠ 0) is called a linear equation in two variables ‘x’ and ‘y’  General form of a linear pair of equations in two variables is: a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0 where a1 b1, c1, a2, b2, c2 are real numbers such that a1 2 + b1 2 ≠ 0 and a2 2 + b2 2 ≠0 LINEAR Equations in two variables: GENERAL FORM OR STANDARD FORM
  5. 5.  The solution of a linear equation in two variables ‘x’ and ‘y’ is a pair of values(one for ‘x’ and other for ‘y’)which makes the two sides of the equation equal.  There are two methods to solve a pair of linear equations: (i) algebraic method (ii) graphical method. SOLUTION OF A PAIR OF LINEAR EQUATIONS IN TWO VARIABLES
  6. 6. (i) If the graphs of two equations of a system intersect at a point, the system is said to have a unique solution, i.e., the system is consistent. (ii) If the graphs of two equations of a system are two parallel lines, the system is said to have no solution, i.e., the system is inconsistent. (iii) When the graphs of two equations of a system are two coincident lines, the system is said to have infinitely many solutions, i.e., the system is consistent and dependent. GRAPHICAL METHOD OF SOLUTION OF A PAIR OF LINEAR EQUATIONS:
  7. 7. Consistent System i) If both the lines intersect at a point, then there exists a unique solution to the pair of linear equations. ii) In such a case, the pair of linear equations is said to be consistent.
  8. 8. If the lines coincide, there are indefinitely many solutions for the pair of linear equations. In this case, each point on the line is a solution. If there are infinitely many solutions of the given pair of linear equations, the equations are called dependent (consistent). Consistent and Dependent Ratio :
  9. 9. Inconsistent system: •If the lines are parallel, there is no solution for the pair of linear equations. •If there is no solution of the given pair of linear equations, the equations are called inconsistent. ≠ c1 c2
  10. 10. GRAPHICAL METHOD OF SOLUTION OF A PAIR OF LINEAR EQUATIONS:
  11. 11. GRAPHICAL METHOD OF SOLUTION OF A PAIR OF LINEAR EQUATIONS:
  12. 12. S
  13. 13. GRAPHICAL METHOD OF SOLUTION OF A PAIR OF LINEAR EQUATIONS:
  14. 14. GRAPHICAL METHOD OF SOLUTION OF A PAIR OF LINEAR EQUATIONS:
  15. 15. GRAPHICAL METHOD OF SOLUTION OF A PAIR OF LINEAR EQUATIONS:
  16. 16. 1. Aftab tells his daughter, “Seven years ago, I was seven times as old as you were then. Also, three years from now, I shall be three times as old as you will be”. Isn’t this interesting? Represent this situation algebraically and graphically. EX:3.1 : SOLUTION : Let the present age of Aftab be ‘x’. And, the present age of his daughter be ‘y’. Now, we can write, seven years ago, Age of Aftab = x – 7 Age of his daughter = y – 7 According to the question, x−7 = 7(y−7) ⇒ x−7 = 7y−49 ⇒ x−7y = −42 ………………………(i)
  17. 17. Also, three years from now or after three years, Age of aftab will become = x + 3. Age of his daughter will become = y + 3 According to the situation given, x+3 = 3(y+3) ⇒x+3 = 3y+9 ⇒x−3y = 6 …………..…………………(ii) Subtracting equation (i) from equation (ii) we have (x−3y)−(x−7y) = 6−(−42) ⇒−3y+7y=6+42 ⇒4y=48 ⇒Y=12 EX:3.1 :
  18. 18. The algebraic equation is represented by x−7y = −42 x−3y = 6 For, x−7y =−42 or x = −42+7y The solution table is For, x−3y=6 or x=6+3y The solution table is EX:3.1 : Eqn (i) x = (-7) (-7) = (-42) + 7y 7y = 42 - 7 = 35 y = 35/7 = 5 Eqn (ii) x = 6 x – 3y = 6 ; 6 – 3y = 6 -3y = 6 – 6 = 0 ; -3y = 0 y = 0
  19. 19. The graphical representation is: EX:3.1 :
  20. 20. 2. The coach of a cricket team buys 3 bats and 6 balls for Rs.3900. Later, she buys another bat and 3 more balls of the same kind for Rs.1300. Represent this situation algebraically and geometrically. EX:3.1 : Let the cost of a bat and a ball be Rs x and Rs y respectively. The given conditions can be algebraically represented as: 3x + 6y = 3900 ; x + 2y = 1300 x + 2y = 1300 .... (i) x + 3y = 1300 .... (ii)
  21. 21. 3. The cost of 2 kg of apples and 1 kg of grapes on a day was found to be ₹160. After a month, the cost of 4 kg of apples and 2 kg of grapes is ₹300. Represent the situation algebraically and geometrically Solution: Let the cost of 1 kg of apples be ‘Rs. x’ And, cost of 1 kg of grapes be ‘Rs. y’ According to the question, the algebraic representation is 2x+y=160 And 4x+2y=300 EX:3.1 :
  22. 22. x 50 60 70 y 60 40 20 x 70 80 75 y 10 -10 0 Three solutions of this equation can be written in a table as follows: For, 2x+y=160 or y=160−2x, the solution table is;
  23. 23. The graphical representation is as follows:EX:3.1 :
  24. 24. (i) Let there are x number of girls and y number of boys. As per the given question, the algebraic expression can be represented as follows. x + y = 10 x – y = 4 Now, for x + y = 10 or x=10−y, the solutions are; x 5 4 6 y 5 6 4 1. Form the pair of linear equations in the following problems, and find their solutions graphically. (i) 10 students of Class X took part in a Mathematics quiz. If the number of girls is 4 more than the number of boys, find the number of boys and girls who took part in the quiz. (ii) 5 pencils and 7 pens together cost Rs. 50, whereas 7 pencils and 5 pens together cost Rs. 46. Find the cost of one pencil and that of one pen. Solution: EX:3.2 :
  25. 25. The graphical representation is as follows; For x – y = 4 or x = 4 + y, the solutions are; x 4 5 3 y 0 1 -1
  26. 26. EX:3.2 : (ii) Let 1 pencil costs Rs.x and 1 pen costs Rs.y. According to the question, the algebraic expression cab be represented as; 5x + 7y = 50 7x + 5y = 46 For, 5x + 7y = 50 or x = {50-7y}/{5} the solutions are; x 3 10 -4 y 5 0 10 (i) If x = 3 x = 50 – 7y/5 3 = 50 – 7y/5 50 – 7y = 15 -7y = 15 - 50 -7y = -35 y = (-35)/(-7) y = 5
  27. 27. x 8 3 -2 y -2 5 12 For 7x + 5y = 46 or x = {46-5y}/{7} the solutions are; (i) If x = 8 x = 46 – 5y / 7 8 = 46 – 5y/7 46 – 5y = 56 -5y = 56 – 46 -5y = 10 y = 10/(-5) y = (-2) EX:3.2 :
  28. 28. Hence, the graphical representation is as follows; From the graph, it is can be seen that the given lines cross each other at point (3, 5). So, the cost of a eraser is 3/- and cost of a chocolate is 5/-. EX:3. 2:
  29. 29. 2. On comparing the ratios , find out whether the lines representing the following pairs of linear equations intersect at a point, are parallel or coincident: (i) 5x – 4y + 8 = 0 : 7x + 6y – 9 = 0 (ii) 9x + 3y + 12 = 0 : 18x + 6y + 24 = 0 (iii) 6x – 3y + 10 = 0 ; 2x – y + 9 = 0 Solutions: EX:3.2 : (i) Given expressions; 5x−4y+8=0 7x+6y−9=0 Comparing these equations with a1x + b1y + c1 = 0 And a2x + b2y + c2 = 0 So, the pairs of equations given in the question have a unique solution and the lines cross each other at exactly one point.
  30. 30. 2. (ii) Given expressions; 9x + 3y + 12 = 0 18x + 6y + 24 = 0 Comparing these equations with a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0 . We get, So, the pairs of equations given in the question have infinite possible solutions and the lines are coincident. EX:3.2 :
  31. 31. (iii) Given Expressions; 6x – 3y + 10 = 0 2x – y + 9 = 0 So, the pairs of equations given in the question are parallel to each other and the lines never intersect each other at any point and there is no possible solution for the given pair of equations. EX:3.2 :
  32. 32. 3. On comparing the ratios a1/a2, b1/b2 & c1/c2 find out whether the following pair of linear equations are consistent, or inconsistent. (i) 3x + 2y = 5 ; 2x – 3y = 7 (ii) 2x – 3y = 8 ; 4x – 6y = 9 (iii)(3/2)x+(5/3)y = 7 ; 9x – 10y = 14 (iv) 5x – 3y = 11 ; – 10x + 6y = –22 (v) (4/3)x+2y = 8; 2x + 3y = 12 EX:3.2 : Solution: (i) Given : 3x + 2y = 5 or 3x + 2y -5 = 0 and 2x – 3y = 7 or 2x – 3y -7 = 0
  33. 33. EX:3.2 : 3x + 2y = 5 ; 2x – 3y = 7
  34. 34. (ii) Given 2x – 3y = 8 and 4x – 6y = 9 Therefore, (iii) (3/2)x+(5/3 EX:3.2 :
  35. 35. (iii) (3/2)x+(5/3)y = 7 and 9x – 10y = 14 Therefore, So, the equations are intersecting each other at one point and they have only one possible solution. Hence, the equations are consistent. EX:3.2 :
  36. 36. (iv) Given, 5x – 3y = 11 and – 10x + 6y = –22 Therefore, EX:3.2 :
  37. 37. (v) (4/3)x+2y = 8 and 2x + 3y = 12 EX:3.2 :
  38. 38. 4. Which of the following pairs of linear equations are consistent/inconsistent? If consistent, obtain the solution graphically: (i) x + y = 5, 2x + 2y = 10 (ii) x – y = 8, 3x – 3y = 16 (iii) 2x + y – 6 = 0, 4x – 2y – 4 = 0 (iv) 2x – 2y – 2 = 0, 4x – 4y – 5 = 0 Solution: (i) Given, x + y = 5 and 2x + 2y = 10 EX:3.2 :
  39. 39. EX:3.2 :
  40. 40. So, the equations are represented in graph as follows: From the figure, we can see, that the lines are overlapping each other. Therefore, the equations have infinite possible solutions. EX:3.2 :
  41. 41. ii) Given, x – y = 8 and 3x – 3y = 16 EX:3.2 :
  42. 42. (iii) Given, 2x + y – 6 = 0 and 4x – 2y – 4 = 0EX:3.2 :
  43. 43. From the graph, it can be seen that these lines are intersecting each other at only one point,(2,2). EX:3.2 :
  44. 44. (iv) Given, 2x – 2y – 2 = 0 and 4x – 4y – 5 = 0 Thus, these linear equations have parallel and have no possible solutions. Hence, the pair of linear equations are inconsistent. EX:3.2 :
  45. 45. 5. Half the perimeter of a rectangular garden, whose length is 4 m more than its width, is 36 m. Find the dimensions of the garden. Solution: Let us consider. Width of the garden is x Length of the garden is y. Now, according to the question, we can express the given condition as; y + x = 36 y – x = 4 Now, taking y – x = 4 or y = x + 4 EX:3.2 :
  46. 46. x 0 8 12 y 4 12 16 Now, taking y – x = 4 or y = x + 4 x 0 36 16 y 36 0 20 For y + x = 36, y = 36 – x EX:3.2 :
  47. 47. The graphical representation of both the equation is as follows; From the graph you can see, the lines intersects each other at a point(16, 20). Hence, the width of the garden is 16 and length is 20. EX:3.2 :
  48. 48. 6. Given the linear equation 2x + 3y – 8 = 0, write another linear equation in two variables such that the geometrical representation of the pair so formed is: (i) Intersecting lines (ii) Parallel lines (iii) Coincident lines Solution: (i) Given the linear equation 2x + 3y – 8 = 0. To find another linear equation in two variables such that the geometrical representation of the pair so formed is intersecting lines, it should satisfy below condition; EX:3.2 :
  49. 49. EX:3.2 : Clearly, you can another equation satisfies the condition. 2x + 3y – 8 = 0 (i) Intersecting lines
  50. 50. (ii) Given the linear equation 2x + 3y – 8 = 0. To find another linear equation in two variables such that the geometrical representation of the pair so formed is parallel lines, it should satisfy below condition; Clearly, you can see another equation satisfies the condition. EX:3.2 : (ii) Parallel lines
  51. 51. iii) Given the linear equation 2x + 3y – 8 = 0. To find another linear equation in two variables such that the geometrical representation of the pair so formed is coincident lines, it should satisfy below condition; Clearly, you can see another equation satisfies the condition. EX:3.2 : (iii) Coincident lines
  52. 52. 7. Draw the graphs of the equations x – y + 1 = 0 and 3x + 2y – 12 = 0. Determine the coordinates of the vertices of the triangle formed by these lines and the x- axis, and shade the triangular region. Solution: Given, the equations for graphs are x – y + 1 = 0 and 3x + 2y – 12 = 0. For, x – y + 1 = 0 or x = 1+y X 0 1 2 Y 1 2 3 EX:3.2 :
  53. 53. For, 3x + 2y – 12 = 0 or x = {12-2y}/{3} X 4 2 0 Y 0 3 6 EX:3.2 : x = 12 – 2(y) 3 (i) y = 0 x = 12 – 2(0) 3 = 12/3 = 4 x = 4. (ii) y = 3 x = 12 – 2(3) 3 = 12 – 6 = 6/3 =2 3 x = 2
  54. 54. Hence, the graphical representation of these equations is as follows; From the figure, it can be seen that these lines are intersecting each other at point (2, 3) and x- axis at (−1, 0) and (4, 0). Therefore, the vertices of the triangle are (2, 3), (−1, 0), and (4, 0). EX:3.2 : X 4 2 0 Y 0 3 6 X 0 1 2 Y 1 2 3
  55. 55. ALGEBRAIC METHOD EX : 3.3
  56. 56. ALGEBRAIC METHOD There are following methods for finding the solutions of the pair of linear equations: 1.Substitution method 2.Elimination method 3.Cross-multiplication method
  57. 57. SUBSTITUTION METHOD
  58. 58. Substitute y = 4 in (3) y = x + 3 4 = x + 3 x = 4 – 3 x = 1. ..(3 )
  59. 59. 1. Solve the following pair of linear equations by the substitution method EX:3.3 :
  60. 60. i) x + y = 14 ... (i) x - y = 4 ... (ii) From (i), we obtain: x = 14 - y ... (iii) Substituting this value in equation (ii), we obtain: Substituting the value of y in equation (iii), we obtain: EX:3.3 :
  61. 61. From (i), we obtain: s = t + 3 ...(iii) Substituting this value in equation (ii), we obtain: Substituting the value of ‘t’ in equation (iii), we obtain: s = 6 + 3 s = 9 ; 2(t+3)+3t=6 6 t = 30/5 EX:3.3 :
  62. 62. EX:3.3 : (iii) Given,
  63. 63. (iv) Given, EX:3.3 : 0.2 x + 0.3y = 1.3 ( multiply by 10) 2x + 3y = 13 ---------(1) 0.4 x + 0.5y = 2.3 ( multiply by 10)’ 4x + 5y = 23 ……….(2) From 1st equation we get, y = 13 – 2x ………….=.(3) 3 Now, substitute the value of y in the given second equation to get, 4x + 5y = 23 4x + 5(13 – 2x/3) = 23 12x + 65 – 10x = 23 x 3 2x + 65 = 96 2x = 69 – 65 2x = 4 x = 4/2 = 2. x = 2 Substitute the value of x in eqn (3) y = 13- 2x/3 = 13 – 2(2) 3 = 13 – 4 3 = 9/3 = 3 y = 3 x = 2, y = 3 iv) 0.2x + 0.3y = 1.3 ... (i) 0.4x + 0.5y = 2.3 ... (ii)
  64. 64. (v)Given:EX:3.3 : ; (-3 )y - √8√2 y =0 √2 -3y-√16y = 0 √2 -3y-4y = 0 √2 -7y = 0 y = 0
  65. 65. (vi)Given:EX:3.3 :
  66. 66. EX:3.3 : 2. Solve 2x + 3y = 11 and 2x – 4y = – 24 and hence find the value of ‘m’ for which y = mx + 3. Solution: -7y = -24 -11 x= 11- 15 = (-4)/2 2
  67. 67. 3. Form the pair of linear equations for the following problems and find their solution by substitution method. (i) The difference between two numbers is 26 and one number is three times the other. Find them. Solution : Let the two numbers be x and y respectively, such that y > x. According to the question, y = 3x ……………… (1) y – x = 26 …………..(2) Substituting the value of (1) into (2), we get 3x – x = 26 x = 13 ……………. ..(3) Substituting (3) in (1), we get y = 39 Hence, the numbers are 13 and 39. EX:3.3 :
  68. 68. (ii) The larger of two supplementary angles exceeds the smaller by 18 degrees. find them. Solution: Let the larger angle by xo and smaller angle be yo. we know that the sum of two supplementary pair of angles is always 180o. According to the question, x + y = 180o……………. (1) x = 18o + y x – y = 18o ……………..(2) from (1), we get x = 180o – y …………. (3) substituting (3) in (2), we get 180o – y – y =18o ; 180o - 18o = -2y 162o = 2y y = 81o ………….. (4) using the value of y in (3), we get x = 180o – 81o = 99o Hence, the angles are 99o and 81o. EX:3.3 :
  69. 69. (iii) The coach of a cricket team buys 7 bats and 6 balls for Rs.3800. Later, she buys 3 bats and 5 balls for Rs.1750. Find the cost of each bat and each ball. Solution: EX:3.3 : Let cost of each bat and each ball be Rs.x and Rs.y respectively. Cost of each bat = Rs. x Cost of each ball = Rs. Y Hence, the cost of each bat and each ball is Rs, 500 and Rs. 50 respectively.
  70. 70. (iv) The taxi charges in a city consist of a fixed charge together with the charge for the distance covered. For a distance of 10 km, the charge paid is Rs.105 and for a journey of 15 km, the charge paid is Rs.155. What are the fixed charges and the charge per km? How much does a person have to pay for travelling a distance of 25 km? EX:3.3 : Solution : Given :
  71. 71. v) A fraction becomes 9/11 , if 2 is added to both the numerator and the denominator. If, 3 is added to both the numerator and the denominator it becomes 5/6. Find the fraction. Solution : Let the required fraction be x/y Then, according to the question, the pair of linear equations formed is , EX:3.3 :
  72. 72. (vi) Five years hence, the age of Jacob will be three times that of his son. Five years ago, Jacob’s age was seven Substituting the value of x in (2), we get 3y + 10 – 7y = -30 -4y = -40 y = 10 ………………… (4) Substituting the value of y in (3), we get x = 3 x 10 + 10 = 40 Hence, the present age of Jacob’s and his son is 40 years and 10 years respectively. EX:3.3 :
  73. 73. elimination method EX
  74. 74. 1. Solve the following pair of linear equations by the elimination method and the substitution method: (i) x + y = 5 and 2x – 3y = 4 (ii) 3x + 4y = 10 and 2x – 2y = 2 (iii) 3x – 5y – 4 = 0 and 9x = 2y + 7 (iv) (x/2)+(2y/3) = -1 and x – (y/3) = 3 EX:3.4 :
  75. 75. Solution: (i) x + y = 5 and 2x – 3y = 4 EX:3.4 : 2x + 2y = 10 2x – 3y = 4 (-) (+) (-) ____________ 0x + 5y = 6 y = 6/5
  76. 76. Elimination method: 3x + 4y = 10 ...(1) 2x - 2y = 2 ...(2) Multiplying equation (2) by 2, we obtain: 4x - 4y = 4 ...(3) adding equation (1) and (3), we obtain EX:3.4 : Solution: (ii) 3x + 4y = 10 2x – 2y = 2 substituting the value of x in equation (1), we obtain: 6 + 4y = 10 4y = 4 y = 1 hence, x = 2, y = 1 3x + 4y = 10 4x – 4y = 4 ____________ 7x + 0y = 14 ____________ 7x = 14 x = 2
  77. 77. Substitution method: Solution: (ii) 3x + 4y = 10 ……………………(1) 2x – 2y = 2 ; x – y = 1………….(2) EX:3.4 :
  78. 78. EX:3.4 : (iii) 3x – 5y – 4 = 0 9x = 2y + 7
  79. 79. (iv) (x/2)+(2y/3) = -1 x – (y/3) = 3 ELIMINATION METHOD : 3x + 4y = - 6 3x – y = 9 (-) (+) (-) ____________ 0x + 5y = -15 ____________ 5y = -15 EX:3.4 :
  80. 80. SUBSTITUTION METHOD: 3x + 4y = -6 ………………..(1) 3x – y = 9 …………………(2) From equation (2), we obtain: y = 3x - 9 …………….... (3) putting this value in equation (1), we obtain: 3x + 4(3x - 9) = -6 15x = 30 x = 2 substituting the value of x in equation (3), we obtain: y = 6 - 9 = -3 Hence x = 2 and y = -3. EX:3.4 : (iv) (x/2)+(2y/3) = -1 x – (y/3) = 3
  81. 81. 2. Form the pair of linear equations in the following problems, and find their solutions (if they exist) by the elimination method: (I) if we add 1 to the numerator and subtract 1 from the denominator, a fraction reduces to 1. It becomes if we only add 1 to the denominator. What is the fraction? Solution: Let the fraction be a/b According to the given information, When equation (i) is subtracted from equation (ii) we get, a = 3 …………………………………………………..(iii) EX:3.4 : When a = 3 is substituted in equation (i) we get, 3 – b = -2 -b = -5 b= 5 Hence, the fraction is 3/5. a – b = - 2 2a – b = 1 (-) (+) (-) ____________ -a + 0y = -3 ____________ a = 3
  82. 82. Let present age of Nuri and Sonu be x and y respectively. According to the question, 2. ii) Five years ago, Nuri was thrice as old as Sonu. Ten years later, Nuri will be twice as old as Sonu. How old are Nuri and Sonu? Solution: Thus, the age of Nuri and Sonu are 50 years and 20 years respectivelySubtracting equation (1) from equation (2), we obtain: y = 20 Substituting the value of y in equation (1), we obtain: x – 3y = -10 x – 3(20) = -10 EX:3.4 : x – 3y = - 10 x – 2y = 10 (-) (+) (-) ____________ 0x -1y = - 20 ____________ y = 20
  83. 83. Adding equations (1) and (2), we obtain: 9y = 9 y = 1 Substituting the value of y in equation (1), we obtain: x + y = 9 x + 1 = 9 x = 9 – 1 = 8. x = 8 Thus, the number is 10y + x = 10 x 1 + 8 = 18 2. (iii) The sum of the digits of a two-digit number is 9. Also, nine times this number is twice the number obtained by reversing the order of the digits. Find the number. EX:3.4 : SOLUTION : x + y = 9 -x + 8y = 0 ____________ 0x +9y = 9 ____________ y = 1 (iii) Let the units digit and tens digit of the number be x and y respectively. Number = 10y + x Number after reversing the digits = 10x + y According to the question, x + y = 9 ... (1) 9(10y + x) = 2(10x + y) 88y - 11x = 0 - x + 8y =0 ... (2)
  84. 84. (Solution: Let the number of Rs.50 notes be A and the number of Rs.100 notes be B. According to the given information, A + B = 25 ………………………(i) 50A + 100B = 2000 A + 2 B = 40…………………….(ii) Subtracting the equation (iii) from the equation (ii) we get, 50B = 750 B = 15 Substituting in the equation (i) we get, A + B = 25 A + 15 = 25 A = 25 – 15 = 10. A = 10 Hence, Manna has 10 notes of Rs.50 and 15 notes of Rs.100. 2. iv) Meena went to a bank to withdraw Rs.2000. She asked the cashier to give her Rs.50 and Rs.100 notes only. Meena got 25 notes in all. Find how many notes of Rs.50 and Rs.100 she received. EX:3.4 : A + B = 25 A + 2B = 40 (-) (-) (-) ____________ 0A - B = -15 ____________ B = 15
  85. 85. Solution: Let the fixed charge for the first three days be Rs.a and the charge for each day extra be Rs.b. According to the information given, a + 4b = 27 ………(i) a + 2b = 21 ……… (ii) (v) A lending library has a fixed charge for the first three days and an additional charge for each day thereafter. Saritha paid Rs.27 for a book kept for seven days, while Susy paid Rs.21 for the book she kept for five days. Find the fixed charge and the charge for each extra day. EX:3.4 : When equation (ii) is subtracted from equation (i) we get, 2b = 6 b = 3………………(iii) Substituting b = 3 in equation (i) we get, a + 12 = 27 a = 15 Hence, the fixed charge is rs.15/- and the charge per day is rs.3/-. a + 4b = 27 a + 2B = 21 (-) (-) (-) ____________ 0a + 2b = 6 ____________ 2b = 6 b = 6/2 = 3
  86. 86. CROSS MULTIPLICATION EX :3.5
  87. 87. b c a b
  88. 88. b c a b
  89. 89. Thus, the given pair of equations has no solution. 1. Which of the following pairs of linear equations has unique solution, no solution, or infinitely many solutions. In case there is a unique solution, find it by using cross multiplication method. (i) x – 3y – 3 = 0 and 3x – 9y – 2 = 0 (ii) 2x + y = 5 and 3x + 2y = 8 (iii) 3x – 5y = 20 and 6x – 10y = 40 (iv) x – 3y – 7 = 0 and 3x – 3y – 15 = 0 Solution: EX:3.5 : (i) x – 3y – 3 = 0 and 3x – 9y – 2 = 0
  90. 90. 1. (ii) 2x + y = 5 and 3x + 2y = 8 EX:3.5 : x y 1 b c a b 1 -5 2 1 2 -8 3 2 x y 1 b1 c1 a1 b1 b2 c2 a2 b2 Thus, the given pair of equations has unique solution. Therefore, It has unique solution. By cross-multiplication,
  91. 91. Thus, the given pair of equations has infinite solutions. 1. (iii) 3x – 5y = 20 and 6x – 10y = 40 EX:3.5 :
  92. 92. Thus, the given pair of equations has unique solution. EX:3.5 : By cross-multiplication, (iv) x – 3y – 7 = 0 and 3x – 3y – 15 = 0 x y 1 b c a b -3 -7 1 -3 -3 -15 3 -3 x y 1 b1 c1 a1 b1 b2 c2 a2 b2 The given pair of equations has unique solution.
  93. 93. EX:3.5 : 2. (i) For which values of a and b does the following pair of linear equations have an infinite number of solutions? 3y + 2x = 7; (a – b) x + (a + b) y = 3a + b – 2 Solution: 2(3a + b - 2) = 7(a-b) 2(a +b) = 3(a-b) 2a -3a = -3b – 2b -a = -5b Therefore , a-5b = 0 a – 5b – ( a – 9b) = 0 – (-4) = - 3b a – 5b –a + 9b = 4 4b = 4
  94. 94. 2. (ii) For which value of k will the following pair of linear equations have no solution? 3x + y = 1; (2k – 1) x + (k – 1) y = 2k + 1 EX:3.5 : 3 ( k-1) = 1 ( 2k – 1) ( cross multiplying) 3k – 2k = -1 + 3
  95. 95. 3. Solve EX:3.5 :
  96. 96. Cross-multiplication method: 3. 8x + 5y = 9; 3x + 2y = 4 EX:3.5 : x y 1 b1 c1 a1 b1 b2 c2 a2 b2 x y 1 b c a b 5 -9 8 5 2 -4 3 2 x = y = 1 -20+18 -27+32 16-15
  97. 97. 4. Form the pair of linear equations in the following problems and find their solutions (if they exist) by any algebraic method: (I) A part of monthly hostel charges is fixed and the remaining depends on the number of days one has taken food in the mess. When a student A takes food for 20 days she has to pay rs.1000 as hostel charges whereas a student B, who takes food for 26 days, pays rs.1180 as hostel charges. Find the fixed charges and the cost of food per day. EX:3.5 : x + 20 y – 1000 = 0 x + 26 y – 1180 = 0 x y 1 b c a b 20 -1000 1 20 26 - 1180 1 26 + + +
  98. 98. , (Ii) A fraction becomes 1/3 when 1 is subtracted from the numerator and it becomes 1/4 when 8 is added to its denominator. Find the fraction. EX:3.5 :
  99. 99. (iii) Yash scored 40 marks in a test, getting 3 marks for each right answer and losing 1 mark for each wrong answer. Had 4 marks been awarded for each correct answer and 2 marks been deducted for each incorrect answer, then Yash would have scored 50 marks. How many questions were there in the test? EX:3.5 :
  100. 100. (iv) Let the speed of first car and second car be u km/h and v km/h respectively. Speed of both cars while they are travelling in same direction = (u - v) km/h (Iv) places a and b are 100 km apart on a highway. One car starts from A and another from B at the same time. If the cars travel in the same direction at different speeds, they meet in 5 hours. If they travel towards each other, they meet in 1 hour. What are the speeds of the two cars? EX:3.5 : Speed of both cars while they are travelling in opposite directions i.e., when they are travelling towards each other = (u + v) km/h Distance travelled = Speed x Time
  101. 101. Substituting the value of u in equation (2), we obtain: u + v = 100 60 + v = 100 v = 100 - 60 v = 40 Hence, speed of the first car is 60 km/h and speed of the second car is 40 km/h. According to the question, EX:3.5 : u – v = 20 u + v = 100 ___________ 2u = 120 ____________ u = 60
  102. 102. (v) Let, The length of rectangle = x unit And breadth of the rectangle = y unit (V) The area of a rectangle gets reduced by 9 square units, if its length is reduced by 5 units and breadth is increased by 3 units. If we increase the length by 3 units and the breadth by 2 units, the area increases by 67 square units. Find the dimensions of the rectangle. EX:3.5 :

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