`a, A = -5/7 . 2/11 + (-5/7) . 9/11 + 1 5/7`
`A = -5/7 . (2/11 + 9/11) + 12/7`
`A = -5/7 . 1 + 12/7`
`A = -5/7 + 12/7`
`A = {-5 + 12}/7`
`A = 7/7`
`A = 1`
-------------------------
`b, B = 6/7 + 5/8 : 5 - 3/16 . (-2)^{2}`
`B = 6/7 + 5/8 . 1/5 - 3/16 . 4`
`B = 6/7 + 1/8 - 3/4`
`B = 48/56 + 7/56 - 42/56`
`B = {48 + 7 - 42}/56`
`B = 13/56`
------------------------
`c, 3 1/3x + 16 3/4 = 13,25`
` 10/3x + 67/4 = 53/4`
` 10/3x = 53/4 - 67/4`
` 10/3x = -7/2`
` x = -7/2 : 10/3`
` x = -7/2 . 3/10`
` x = -21/20`
$\text{Vậy}$ ` x = -21/20`
----------------------------
`d, 2/{3.5} + 2/{5.7} +...+ 2/{13.15}`
`= 1/3 - 1/5 + 1/5 - 1/7 +...+ 1/13 - 1/15`
`= 1/3 - 1/15`
`= 4/15`
