Đặt `a/b=c/d=k`
Ta có :
`a=bk;c=dk`
`⇒ [3a+4b]/[3a-4b]=[3bk+4b]/[3bk-4b]=[b(3k+4)]/[b(3k-4)]=[3k+4]/[3k-4]` `(1)`
`⇒ [3c+4d]/[3c-4d]=[3dk+4d]/[3dk-4d]=[d(3k+4)]/[d(3k-4)]=[3k+4]/[3k-4]` `(2)`
Từ `(1),(2) => [3a+4b]/[3a-4b]=[3c+4d]/[3c-4d]` (ĐPCM)
Học tốt !