（1-x^2)(1-y^2)+4xy
=1-x²-y²+x²y²+4xy
=（x²y²+2xy+1）-（x²-2xy+y²）
=(xy+1)²-(x-y)²
=(xy+1+x-y)(xy+1-x+y)
分解因式：（1-x^2)(1-y^2)+4xy
（1-x^2)(1-y^2)+4xy
=1-x²-y²+x²y²+4xy
=（x²y²+2xy+1）-（x²-2xy+y²）
=(xy+1)²-(x-y)²
=(xy+1+x-y)(xy+1-x+y)