d/dx是对x求导
dy/dx是y对x的导数,即y'
d^2y/dx^2即d/dx(dy/dx),是y'对x的导数,即y'',是二阶导数
例：y=x^2
dy/dx=(x^2)'=2x
d(x^2)/dx=2x
d^2y/dx^2=(2x)'=2