tan(9π/4+a)*tan(3π/4+a)=tan(2π+π/4+a)*tan(π-π/4+a)= tan(a+π/4)*tan(a-π/4)=(1+tana)/(1-tana)*(1-tana)/(1+tana)=1 sina=sin(a+30-30)=√3/2*sin(a+30)-1/2cos(a+30)60＜a＜90 90＜a+30＜120cos(a+30)=-...已知tan(π/4-a)=1/3 求{2cosa(sina-cosa)}/1+tana由tan(π/4-a)=1/3得tana=1/2{2cosa(sina-cosa)}/1+tana=[2cos^2a(1/2-1)/(1+1/2)=-2/3cos^2atana=1/2构成一直角三角形得cos^2a=4/5所以-2/3cos^2a=-8/15