`a, 75/90+26/24`

Rút gọn: `75/90 = (75 : 15)/(90 : 15) = 5/6 ; 26/24 = (26 : 2)/(24 : 12) = 13/12`

`5/6 + 13/12 = 10/12 + 13/12 = 23/12`

Vậy: `75/90 + 26/24 = 23/12`

`b, 42/49-38/56`

Rút gọn: `42/49 = (42 : 7)/(49 : 7) = 6/7 ; 38/56 = (38 : 2)/(56 : 2) = 19/28`

`6/7 - 19/28 = 24/28 - 19/28 = 5/28`

Vậy: `42/49 - 38/56 = 5/28`

`c, 51/72 + 55/66`

Rút gọn: `51/72 = (51 : 3)/(72 : 3) = 17/24 ; 55/66 = (55 : 11)/(66 : 11) = 5/6`

`17/24 + 5/6 = 17/24 + 20/24 = 37/24`

Vậy: `51/72+55/66 = 37/24`

`d,1313/1515-20/25`

Rút gọn: `1313/1515 = (1313 : 101)/(1515 : 101) = 13/15 ; 20/25 = (20 : 5)/(25 : 5) = 4/5`

`13/15 - 4/5 = 13/15 - 12/15 = 1/15`

Vậy: `1313/1514 - 20/25 = 1/15`