若2分之x=3分之y,求x2-6xy-7y2分之x2-2xy-3y2
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x/2=y/3

x/y=2/3

(x^2-2xy-3y^2)/(x^2-6xy-7y^2)分子分母同时除以y^2

=(x^2/y^2-2xy/y^2-3y^2/y^2)/(x^2/y^2-6xy/y^2-7y^2/y^2)

=[(x/y)^2-2x/y-3]/[(x/y)^2-6x/y-7]

=[(2/3)^2-2*2/3-3]/[(2/3)^2-6*2/3-7]

=(4/9-4/3-3)/(4/9-4-7)

=(4/9-12/9-27/9)/(4/9-99/9)

=(-35/9)/(-95/9)

=35/95

=7/19

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