y = tan（x/2）y ′ = sec²(x/2) * 1/2 = (1/2) sec²(x/2) y ′′ = (1/2) * 2sec(x/2) * sec (x/滚漏老大升2) tan (x/2) * 1/2= (1/2) sec²(x/2) tan (x/搜搜2)
