e^(x^2+2y) + xy^2 = 1两边对 x 求导， 得 e^(x^2+2y)(2x+2y') + y^2 + 2xyy' = 0解得 y' = [-2xe^(x^2+2y)-y^2]/[2e^(x^2+2y)+2xy]
