Question

sin a+cos a=五分之一,求sin a乘以cos

Answer 1

(sina+cosa)^2=1/25

1+2sinacosa=1/25

sinacosa=-12/25

sina^4-cosa^4

(sina^2-cosa^2)(sina^2+cosa^2)

=(sina-cosa)(sina+cosa)*1

=-12/25*1/5

=-12/125