通常令y=kx(方向)或x^n or 缩放1y^2=zxz/(x^2+z^2)与3同类2|xy^2/(x^2+y^2)|<=|xy^2/(y^2)|<=|x| ->04|xy^3/(x^2+y^2)|<=|xy^3/(y^2)|<=|xy| ->03令x=y，极限=1/2令x=2y, 极限=2/5令y=3x, 极限=3/10x=0 or y=0, 极限=0何405的极坐标方法也很好
