(sinα)^2-3sinαcosα+1=[(sinα)^2-3sinαcosα+(sinα)^2+(cosα)^2]/[(sinα)^2+(cosα)^2]=[2(sinα)^2-3sinαcosα+(cosα)^2]/[(sinα)^2+(cosα)^2] 分子分母同除(cosα)^2=[2(tanα)^2-3tanα+1]/[(tanα)^2+1]=(8-6+1)/(4+1)=3/5
