Sửa đề do sai:
Đặt `a/b=c/d=k⇒a=bk; c=dk`
`(2015a-2016b)/(2016c+2017d)=(2015a-2016b)/(2016c+2017d)`
`⇒(2015bk-2016b)/(2016dk+2017d)=(2015bk-2016b)/(2016dk+2017d)`
`⇒b/d .(2015k-2016)/(2016k+2017)=b/d .(2015k-2016)/(2016k+2017)`
`⇒b/d=b/d` (luôn đúng)
`⇒Đpcm`