Đáp án:
`a) 1/3 - 5/7 : -3/15`
`= 1/3 - 5/7 . 15/-3`
`= 1/3 - -25/7`
`= 1/3 + 25/7`
`= 82/21`
`b) (-5)/7 . 4/13 + 9/13 . (-5)/7 + 1 3/7`
`= (-5)/7 . 4/13 + 9/13 . (-5)/7 + 10/7`
`= (-5)/7 . (4/13 + 9/3) + 10/7`
`= (-5)/7 . 1 + 10/7`
`= (-5)/7 + 10/7`
`= 5/7`