分解下列因式:(1)m2-n2+2m-2n (2)1-x2-y2+2xy(3)
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(1)m2-n2+2m-2n
=(m+n)(m-n)+2(m-n)
=(m+n)(m-n+2);
(2)1-x2-y2+2xy
=1-(x2+y2-2xy)
=1-(x-y)2
=(1+x-y)(1-x+y);
(3)x2-(k+3)x+(k+2)
=(x-2-k)(x-1);
(4)(m2-1)(n2-1)+4mn
=m2n2-n2-m2+1+4mn
=m2n2+2mn+1-(n2+m2-2mn)
=(mn+1)2-(m-n)2
=(mn+1-m+n)(mn+1+m-n).