tana=2
则tan(π/4+a)=(tan(π/4)+tana)/(1-tan(π/4)*tana)=(1+2)/(1-1*2)=-3
tan(2a)=2tana/(1-(tana)^2)=2*2/(1-2^2)=-4/3
所以sin(π/4+a)/cos(π/4+a)*tan(2a)
=tan(π/4+a)*tan(2a)
=-3*(-4/3)
=4