Question

已知sin阿尔法=1/3,阿尔法属于(0,派),则cos(派+2阿尔法)等于?

Answer 1

cos(派+2阿尔法)

=-cos2阿尔法

=-(1-2sin^2 a)

=2sin^2 a-1

=2*(1/3)^2-1

=2/9-1

=-7/9