求隐函数y的二阶导数d^2y/dx^2 y=tan(x+y)
问题描述：
求隐函数y的二阶导数d^2y/dx^2 y=tan(x+y)
答
y‘=y'sec²(x+y)
y‘=1/[1-sec²(x+y)]
y''=-2y'sec²(x+y)tan(x+y)/[1-sec²(x+y)]²
=-2sec²(x+y)tan(x+y)/[1-sec²(x+y)]^3