1 .原式= (sin²a +cos ²a) (sin²a-cos²a)=sin²a-cos²a=-(cos²a-sin²a)=-cos2a=1-2sin²a=1-0.4=0.6 故选C
2
cos 2a= cos²a-sin²a=(cos a + sin a)(cos a - sin a)
(sin a+cos a)²=(1/5)² ==> 2sin a *cos a= -24/25
(cos a - sin a)²=1-2sin a*cos a = 73/25
因为a属于[90,135]
所以 cos a - sin a= - 根号（73）/5
原式=1/5 * - 根号（73）/5 = - 根号（73）/25
3
sin2a=2 sin a * cos a = -24/25
(sin a+cos a)²=(1/5)² ==> 2sin a *cos a =-24/25