（3x-y+1）（2x+y-1）=｛3x-（y-1）｝（2x+y-1）と変形し、y-1をAと置く。
（3x-A）（2x+A）=6x^2+xA-A^2
=6x^2+x（y-1）-（y-1）^2
=6x^2+xy-x-（y^2-2y+1）
=6x^2+xy-x-y^2+2y-1
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