求极限 lim(x→ +∞,y→ +∞) (x^2+y^2)/(x^4+y^4)?

求极限lim(x,y)→(+∞,+∞) (xy/(x^2+y^2))^x^2 求极限lim(x,y)→(+∞,+∞) [(xy)/(x^2+y^2)]^xy. lim(x→0y→1)(1+xe^y)^(2y+x/x)求极限 多元函数极限求解lim(x→∞,y→∞)((x+y)/(x^2+xy+y^2))=? 求极限lim(x→1 y→2) (x²+y²)/xy 求下列各极限 lim(x,y)→(0,1) (2-xy)/(x^2+2y) 二元函数求极限：lim sin（x^2+y）/(x^2+y^2) x→0,y→0 二元函数求极限：lim (sin（x^2+y）) / (x^2+y^2) x→0,y→0 求极限 lim x→0 y→0[x^2+y^2+5/x+y sin(x+y)] 不懂极限, 求一个二元函数的极限lim((x^2)y+x^5)/(x^4+x^6+2(x^3)y+y^2))(x,y)->(0,0) lim((x-y)/(x+y))求极限.(x,y)→(0,0) 跪求极限Y=lim (xy+1)/x^4+y^4,当（x,y）→（0,0）,