化简(sin^2a-tan^2a)*(sin^2a*tan^2a)=(1-cos^2a-tan^2a)*[(1-cos^2a)tan^2a]=[1-1/(1+tan^2a)-tan^2a]*[1-1/(1+tan^2a]tan^2a=[tan^2a/(1+tan^2a)-tan^2a][tan^2a/(1+tan^2a]tan^2a