Question

21/(x+y)+4/(x-y)=12/(x+y)+7/(x-y)

Answer 1

21/(x+y)+4/(x-y)=12/(x+y)+7/(x-y)

移项后得：

21/(x+y)-12/(x+y)= 7/(x-y) -4/(x-y)

即：9/(x+y)= 3/(x-y)

因此：(x+y)/(x-y)=3