x的平方-2xy-3y的平方/x的平方-6xy-7y的平方的值
=(x²-2xy-3y²)/(x²-6xy-7y²)
=(x-3y)(x+y)/(x-7y)(x+y)
=（x-3y）/(x-7y)
=(x/y-3)/(x/y-7)
=(-2-3)/(-2-7)
=5/9;
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