tan(派/4+ x)=(1+tanx)/(1-tanx)=(cosx+sinx)/(cosx-sinx)=3cosx+sinx=3cosx-3sinx 2sinx=cosxtanx=1/2sin2x-2cos^2x=2sinxcosx-2cos^2x=-cos^2xtanx^2+1=secx^2=1/cosx^2所以原式=-1/(tanx^2+1)=-4/5