`\text{14)}`
`420 : [65 - (x-10 )] = 21`
`-> 65 - (x - 10) = 420 : 21`
`-> 65 - (x -10) =20`
`-> x - 10 = 65 - 20`
`-> x - 10 = 45`
`-> x = 45 +10`
`-> x = 55`
Vậy `x =55`
`\text{c)}}`
`(2/35 + 4/77 +2/143 + 4/221 +2/323 +4/437 +2/575) : x = 1/2 +1/6 +1/12 + ... +1/110 +1/132)`
`-> ( 2/{5 × 7} + 4/{7×11} + 2/{11×13} + 4/{13×17} + 2/{17×19} +4/{19 ×23} + 2/{23 ×25} ) : x = 1/{1 × 2} + 1/{2×3} + 1/{3×4} + ... + 1/{ 10 ×11} + 1/{11×12}`
`-> ( 1/5 - 1/7 + 1/7 -1/11 +1/11 - 1/13 + 1/13 - 1/17 + 1/17 -1/19 + 1/19 -1/23 +1/23 -1/25) : x = 1 - 1/2 +1/2 -1/3 +1/3 -1/4 + ... + 1/10 -1/11 + 1/11 -1/12)`
`-> [ 1/5 + (1/7 -1/7) + (1/11 -1/11) + (1/13 -1/13) + (1/17 -1/17) + (1/19 -1/19) + (1/23-1/23) - 1/25 ] : x = 1 + (1/2 -1/2) + (1/3 -1/3) + (1/4 -1/4) + ... + (1/11 -1/11) -1/12`
`-> ( 1/5 - 1/25 ) : x =1 -1/12`
`-> ( 5/25 -1/25) : x = 12/12 -1/12`
`-> 4/25 : x = 11/12`
`-> x = 4/25 : 11/12`
`-> x = 4/25 . 12/11`
`-> x = 48/275`
Vậy `x = 48/275`
