令y=x-k
lim(x→+∞,y→+∞) (X+Y)/(X^2-Y^2)
=lim(x→+∞,y→+∞) [1/(X-Y)]
=lim(x→+∞,y→+∞) [1/(X-(X-k))]
=1/k
此时极限是一个与K相关的不定值,所以极限不存在