f(x)=2*2tan(x/2)/[1-tan²(x/2)]+2*[1-2sin²(x/2)]/[2sin(x/2)cos(x/2)]=2tanx+2cosx/sinx=2(sinx/cosx+cosx/sinx)=2(sin²x+cos²x)/sinxcosx=4*1/(2sinxcosx)=4/sin2x所以f(π/12)=4/sin(π/6)=8
