1. Exercise #17
Sum and Difference Identities II
 
Express cos   x  as a function of x only.
2 
Therefore, we can use the sum and difference identities when proving identities.
Example
  1 3
Prove sin  x    sin x  cos x .
 3 2 2
Working Backwards
Evaluate without using a calculator:
 5  5
sin cos  cos sin
9 36 9 36

2. Try
Find the exact value of:
   
cos cos  sin sin
12 3 12 3
Example
Prove that sin(   )  sin(   )  2cos  sin  .
Journal
4 5 3
Given that tan    where  is not in quadrant IV and cot   where     ,
3 12 2
find cos(   ) .