∵tana=2∴tan2a=2tana/(1-tan²a)=4/(1-4)=-4/3∴（sin2a-cos2a）/（sin2a+cos2a）=(sin2a/cos2a-1)/(sin2a/cos2a+1) 【分子分母同时除以cos2a,化切】=(tan2a-1)/(tan2a+1)=(-4/3-1)/(-4/3+1)=7
