This document discusses sum and difference formulas for trigonometric functions. It provides examples of finding the exact values of trigonometric functions like sin(75°), tan(105°), and cos(11π/12). It also gives examples of using the sum and difference identities, like evaluating cos(78°)cos(18°) + sin(78°)sin(18°) and verifying trigonometric equations. Students are assigned problems from page 404 involving applying sum and difference formulas to evaluate trigonometric functions without a calculator.