Bài `2 :`
`c, x : [ ( 1800 + 60 ) : 30 ] = 560 : ( 315 - 35 )`
`⇔ x : [ 1860 : 30 ] = 560 : 280`
`⇔ x : 62 = 2`
`⇔ x = 2 . 62`
`⇔ x = 124`
Vậy `, x = 124 .`
`d, ( x + 1 ) + ( x + 4 ) + ( x + 7 ) + .... + ( x + 100 ) = 1887`
`⇔ ( x + x + .... + x ) + ( 1 + 4 + 7 + .... + 100 ) = 1887`
`⇔ 34 . x + ( 100 + 1 ) . 34 : 2 = 1887`
`⇔ 34 . x + 101 . 17 = 1887`
`⇔ 34 . x + 1717 = 1887`
`⇔ 34 . x = 1887 - 1717`
`⇔ 34 . x = 170`
`⇔ x = 170 : 34`
`⇔ x = 5`
Vậy `, x = 5 .`
Bài `3 :`
`a,` Ta có `: 3ab + 193 = ab7`
`⇔ 300 + a . 10 + b + 193 = a . 100 + b . 10 + 7`
`⇔ 486 = 90 . a + 9 . b`
`⇔ 486 = 9 . 10 . a + 9 . b`
`⇔ 486 = 9 . ( 10 . a + b )`
`⇔ 486 : 9 = 10 . a + b`
`⇔ 54 = ab`
Vậy `, a = 5 ; b = 4 .`
`b,` Ta có `: 260abc = 626 . abc`
`⇔ 260abc : abc = 626`
`⇔ ( 260000 + abc ) : abc = 626`
`⇔ 260000 : abc + abc : abc = 626`
`⇔ 260000 : abc + 1 = 626`
`⇔ 260 000 : abc = 626 - 1`
`⇔ 260 000 : abc = 625`
`⇔ abc = 260 000 : 625`
`⇔ abc = 416`
Vậy `, a = 4 ; b = 1 ; c = 6 .`