cos 2A has THREE forms: cos^2A - sin^2A = 2cos^2A - 1 = 1 - 2sin^2A. The last two forms are used to derive half-angle formulas: cos^2(A/2) = (1 + cosA)/2 and sin^2(A/2) = (1 - cosA)/2. Choose the form based on what needs to be eliminated.