隐函数求导公式
xy² - e^xy + 2 = 0 ；y² + 2xyy′ - e^xy(y + xy′) = 0 ；y² + 2xyy′ - ye^xy - xy′e^xy = 0 ；(2xy - xe^xy)y′ = ye^xy - y² ；因此 y′ = dy/dx = y（e^xy - y0）/ x(2y - e^xy)
xy² - e^xy + 2 = 0 ；y² + 2xyy′ - e^xy(y + xy′) = 0 ；y² + 2xyy′ - ye^xy - xy′e^xy = 0 ；(2xy - xe^xy)y′ = ye^xy - y² ；因此 y′ = dy/dx = y（e^xy - y0）/ x(2y - e^xy)
