`a) 7 . (x + 25) - 20 = 190`
` 7 . (x + 25) = 190 + 20`
` 7 . (x + 25) = 210`
` x + 25 = 210 : 7`
` x + 25 = 30 `
` x = 30 - 25`
` x = 5`
`b) 50 - [(4x + 25) : 3 - 810 : 2 . 9] = 48`
` (4x + 25) : 3 - 405 . 9 = 50 - 48`
` (4x + 25) : 3 - 3645 = 2`
` (4x + 25) : 3 = 2 + 3645`
` (4x + 25) : 3 = 3647`
` 4x + 25 = 3647 . 3`
` 4x + 25 = 10941`
` 4x = 10941 - 25`
` 4x = 10916`
` x = 10916 : 4`
` x = 2729`
`c) 25; 26; 27; ... ; x`
Số số hạng của dãy là:
` (x - 25) : 1 + 1 = x - 24`
`25 + 26 + 27 + ... + x = 4750`
`(x + 25) . (x - 24) : 2 = 4750`
`(x + 25) . (x - 24) = 4750 . 2`
`(x + 25) . (x - 24) = 9500`
`xx - x24 + 25x - 600 = 9500`
`xx - x = 9500 + 600`
`x(x - 1) = 10100`
`x(x - 1) = 101 . 100`
`⇒ x = 101`
