f(tanx/2)=sinx
f(tanx/2)=2(tanx/2)/[1+(tanx/2)^2]
f(x)=2x/(1+x^2)
f(x)=2x/(1+x^2)=2/(x+1/x)
由x+1/x>=2*√(x*1/x)=2,
故f(x)