原式=(x+y)²-[4(x-y)]²
=(x+y)²-(4x-4y)²
=[(x+y)+(4x-4y)][(x+y)-(4x-4y)]
=(5x-3y)(5y-3x)
(x+y)²-16（x-y）²=[x+y-4(x-y)]×[x+y+4(x-y)]=(5y-3x)(5x-3y) 就是这样了......
用平方差公式.....16=4²
（x+y）^2-16（x-y）^2
=[(x+y)-4(x-y)][(x+y)+4(x-y)]
=(5y-3x)(5x-3y)
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