若2分之x=3分之y,求x2-6xy-7y2分之x2-2xy-3y2

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