解答;
(X+Y)^2-9Y^2=(x+y+3y)(x+y-3y)
=(x+4y)(x-2y)
（X+Y)^2-（X-Y）^2
=x^2+2xy+y^2-x^2+2xy-y^2=4xy
（X-Y)^2-81X^2Y^2=(x-y+9xy)(x-y-9xy)
n^3（M-2)-N(M-2)
=n(m-2)(n^2-1)=n(m-2)(n+1)(n-1)
4(a-B）^2-9（a+b）^2
=(2a-2b+3a+3b)(2a-2b-3a-3b)
=-(5a+b)(a+5b)
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