已知tana=根号2,2sin²a-sinacosa+cos²a-1

tana=根号2; sin²a=4/5,cos²a=1/5
2sin²a-sinacosa+cos²a-1=sin²a-sinacosa=sin²a(1-1/tana)=4/5*(1-1/根号2)
=(2-根号2) * 2/5