tanx=2tan(x/2)/(1-tan^2x)=-4/3cos^4x-sin^4x=(cos^2x+sin^2x)(cos^2x-sin^2x)=cos^2x-sin^2xcos(π/4+x)=cosπ/4cosx-sinπ/4sinx=√2/2(cosx-sinx)原式=(cos^2x-sin^2x)/(cosx-sinx)sinx=(1-tan^2x)/(tanx-tan^2)...=(1-tan^2x)/(tanx-tan^2)这一步是不是有点问题？？分子分母同除以cos^2x