Bài `1 :`
`a, ( - 3 )/4 + 3/7 + ( - 1 )/4 + 4/9 + 4/7`
`= ( ( - 3 )/4 + ( - 1 )/4 ) + ( 3/7 + 4/7 ) + 4/9`
`= - 1 + 1 + 4/9`
`= 0 + 4/9`
`= 4/9`
`b, ( - 7 )/9 . 4/11 + ( - 7 )/9 . 7/11 + 5 7/9`
`= ( - 7 )/9 . ( 4/11 + 7/11 ) + 5 7/9`
`= ( - 7 )/9 . 1 + 5 7/9`
`= ( - 7 )/9 + 5 7/9`
`= 5`
`c, ( 21/31 + 2013/6039 ) - ( 44/53 - 10/31 ) - 9/53`
`= 21/31 + 1/3 - 44/53 + 10/31 - 9/53`
`= ( 21/31 + 10/31 ) - ( 44/53 + 9/53 ) + 1/3`
`= 1 - 1 + 1/3`
`= 0 + 1/3`
`= 1/3`
