已知cosα=-3/5,且π
问题描述：
已知cosα=-3/5,且π
答
cos2α
=cos(α+α)
=cosαcosα-sinαsinα
=cos²α-sin²α
=cos²α-(1-cos²α)
=2cos²α-1
=2×(-3/5)² -1
=18/25-1
=-7/25