因为cosα=cos(α/2+α/2)=cos(α/2)*cos(α/2)-sin(α/2)*sin(α/2)=1-2sin(α/2)*sin(α/2)所以sin(α/2)*sin(α/2)=(1-cosα)/2已知sinα=a,α为锐角,所以有cosα>0,α/2为锐角,sin(α/2)>0,所以cosα=根号下(1-s...