In Exercise 13b, you developed the formula cos2 A = 1+cos2A/2. Use this formula to develop the half-angle formula for cosine. Begin by taking the square root of both sides, then substitute θ/2 for A. Use the formula from Exercise 13a for sin2 A and a similar process to develop the half-angle formula for sine. Then use the half-angle formulas for sine and cosine to develop the half-angle formula for tangent.
In Exercise 13b
Use another double-angle identity to develop an expression equivalent to cos2A that does not contain sin2A.