`1,`
`a, 1/2 - 1/2 . 4/5 + 4/5 . 5/6`
`= 1/2 - 4/5 . ( 1/2 + 5/6 )`
`= 1/2 - 4/5 . 4/3`
`= 1/2 - 16/15`
`= ( - 17 )/30`
`2,`
`a, x - 1/2 = 0,75`
`⇔ x - 1/2 = 3/4`
`⇔ x = 3/4 + 1/2`
`⇔ x = 5/4`
Vậy `, x = 5/4 .`
`b, 5x + 4/5 = 0,5 + 25%`
`⇔ 5x + 4/5 = 1/2 + 1/4`
`⇔ 5x + 4/5 = 3/4`
`⇔ 5x = 3/4 - 4/5`
`⇔ 5x = ( - 1 )/20`
`⇔ x = ( - 1 )/20 : 5`
`⇔ x = ( - 1 )/100`
Vậy `, x = ( - 1 )/100 .`