`( x - 4 )/10 + ( x - 5 )/11 = ( x - 6 )/12 + ( x - 7 )/13`
`⇔ ( ( x - 4 )/10 + 1 ) + ( ( x - 5 )/11 + 1 ) = ( ( x - 6 )/12 + 1 ) + ( ( x - 7 )/13 + 1 )`
`⇔ ( x + 6 )/10 + ( x + 6 )/11 = ( x + 6 )/12 + ( x + 6 )/13`
`⇔ ( x + 6 )/10 + ( x + 6 )/11 - ( x + 6 )/12 - ( x + 6 )/13 = 0`
`⇔ ( x + 6 ) . ( 1/10 + 1/11 - 1/12 - 1/13 ) = 0`
`⇔ x + 6 = 0`
`⇔ x = 0 - 6`
`⇔ x = - 6`
Vậy `, x = - 6 .`