已知x/2=y/3（xy≠0）,求x²-2xy-3y²/x²-6xy-7y²的值
问题描述：
已知x/2=y/3（xy≠0）,求x²-2xy-3y²/x²-6xy-7y²的值
答
x/2=y/3
x=2y/3
所以
x²-2xy-3y²/x²-6xy-7y²
=(x+y)(x-3y)/(x+y)(x-7y)
=(x-3y)/(x-7y)
=(2y/3-3y)/(2y/3-7y)
=(2-9)/(2-21)
=7/19