Đặt `A = a - 1/4 + a - 1/4 + a - 1/4 + b - 1/8 + b - 1/8 + b - 1/8`
` = (a- 1/4) + (a-1/4) + (a-1/4) + (b-1/8) + (b-1/8) + (b-1/8)`
` = 3 . (a-1/4) + 3 . (b-1/8)`
` = 3 . (a - 1/4 + b - 1/8)`
Vì `a = 1/2 - b` nên :
`A = 3 . (1/2 - b - 1/4 + b - 1/8)`
` = 3 . [(b-b) + (1/2 - 1/4 -1/8)]`
` =3 . ( 0 + 4/8 - 2/8 - 1/8)`
` = 3 . 1/8`
` = 3/8`