三角函数的题 已知cosa=3/5,a为锐角,求1-2sin² (π/8-π/2)好像不对啊 我这里的答案

1-2sin² (π/8-π/2)
=1-2sin² (-3π/8)
=1-2sin² (3π/8)
=cos(3π/4)
=-cos(π/4)
=-(根2)/2