Power reduction formula: cos²θ = (1 + cos 2θ)/2
cos²θ = (1 + cos 2θ)/2 - the cosine counterpart of the sine power-reduction formula, differing only in the sign. Applied repeatedly it reduces any even power of sine or cosine down to first-degree terms.
Related Power Reduction identities
Common questions
What is the cos²θ identity?
cos²θ = (1 + cos 2θ)/2 - the cosine counterpart of the sine power-reduction formula, differing only in the sign. Applied repeatedly it reduces any even power of sine or cosine down to first-degree terms.
Which identities are related to cos²θ?
cos²θ belongs to the Power Reduction group. The identities used alongside it most often are sin²θ.
Practise it
Reading a formula is not the same as recalling it under time pressure. The workspace drills Power Reduction questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.
Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.