y=lntanx/2的导数
令y=lnu
u=tan(x/2)
v=x/2
y=(lnu)=(1/u)*(u)=(1/u)*(tanv)*(v)
代入原函数
y\'={1/tan(x/2)}*{sec^2(x/2)}*(1/2)
y\'=cot(x/2)*sec^2(x/2)*(1/2)
y\'=1/{2sin(x/2)*cos(x/2)}=1/sinx
所以：y\'=1/sinx
(f(x)+/-g(x))\'=f\'(x)+/- g\'(x)
(f(x)g(x))\'=f\'(x)g(x)+f(x)g\'(x)
(g(x)/f(x))\'=(f(x)\'g(x)-g(x)f\'(x))/(f(x))^2
