x^2/(x^2+9y^2)So 所以0≤|(x^2 * sin^2y)/(x^2 +9y^2)-0|=|x^2/(x^2+9y^2)|*sin^2 y≤1*sin^2 y=sin^2y取极限可得sin^2y->0所以由夹逼法则，lim(x,y)->(0,0) (x^2 * sin^2y)/(x^2 +9y^2) =0
