tana/(tana-1)=-1,求2(sina)^2-sina*cosa+(cosa)^2

tana/(tana-1)=-1,tana=1/22(sina)^2-sina*cosa+(cosa)^2=[2(sina)^2-sina*cosa+(cosa)^2]/[(sina)^2)+(cosa)^2]=(2 tan^2a- tana+1)/( tan^2a+1)原式＝(2*1/4-1/2+1)/(1/4+1)=4/5所以,2(sina)^2-sina*cosa+(cosa)^2=4/5