~ Bạn tham khảo ~
Đặt `a/b=c/d=k`
`=> a=bk;c=dk`
Thay vào ta có :
`[4a+3c]/[4b+3d] = [4bk+3dk]/[4b+3d] = [k(4b+3d)]/[4b+3d] = k` `(1)`
`[4a - 3c]/[4b - 3d] = [4bk-3dk]/[4b-3d] = [k(4b-3d)]/[4b-3d] = k` `(2)`
Từ `(1),(2)=> [4a+3c]/[4b+3d]=[4a-3c]/[4b-3d]` (Đpcm)