分子有理化
(√(1+x²y²)-1)/(x²+y²)
=(1+x²y²-1)/[(x²+y²)(√(1+x²y²)+1)]
=x²y²/[(x²+y²)(√(1+x²y²)+1)]
显然x->0,y->0时
√(1+x²y²)+1->1
x²y²/[(x²+y²)->0
所以lim(x→0,y→0) [ (√(1+x²y²)-1)/(x²+y²) ]=0
分子有理化
(√(1+x²y²)-1)/(x²+y²)
=(1+x²y²-1)/[(x²+y²)(√(1+x²y²)+1)]
=x²y²/[(x²+y²)(√(1+x²y²)+1)]
显然x->0,y->0时
√(1+x²y²)+1->1
x²y²/[(x²+y²)->0
所以lim(x→0,y→0) [ (√(1+x²y²)-1)/(x²+y²) ]=0