前面你看下是 X-3Y还是X-4Y
3x/(x-3y)+(x+y)/(4y-x)-7y/(x-4y)
=3x/(x-3y)-(x+y)/(x-4y)-7y/(x-4y)
=3x(x-3y)-(x+y-7y)/(x-4y）
=3x(x-3y)-(x-6y)/(x-4y)

3x/(x-4y)+(x+y)/(4y-x)-7y/(x-4y)
=3x/(x-4y)-(x+y)/(x-4y)-7y/(x-4y)
=(3x-x-y-7y)/(x-4y)
=(2x-8y)/(x-4y)=2