$\\$
Bài `5.`
`a,`
`51^3`
`= (50+1)^3`
`= 50^3 + 3 . 50^2 . 1 + 3 . 50 . 1^2 + 1^3`
`= 132651`
`b,`
`89^3 + 33 . 89^2 + 3 . 121 . 89 + 11^3`
`= 89^3 + 3 . 89^2 . 11 + 3 . 89 . 11^2 + 11^3`
`= (89 + 11)^3`
`= 100^3`
`= 100 000`
`c,`
`23^3 - 9 . 23^2 + 27 . 23 - 27`
`= 23^3 - 3 . 23^2 . 3 + 3 . 23 . 3^2 - 3^3`
`= (23 - 3)^3`
`= 20^3`
`= 8000`
$\\$
Bài `6.`
`a,`
`(x+2)^3`
`=x^3 + 3 . x^2 . 2 + 3 . x . 2^2 +2^3`
`= x^3 + 6x^2 + 12x + 8`
`b,`
`(x-1/2)^3`
`=x^3 - 3 . x^2 . 1/2 + 3 . x . (1/2)^2 -(1/2)^3`
`=x^3 - 3/2x^2 + 3/4 x - 1/8`
`c,`
`(x-2y)^3`
`= x^3 - 3 . x^2 . 2y + 3 . x . (2y)^2 - (2y)^3`
`= x^3 - 6x^2y + 12xy^2 - 8y^3`
`d,`
`(x + y^2/2)^3`
`= x^3 + 3 . x^2 . y^2/2 + 3 . x . (y^2/2)^2 + (y^2/2)^3`
`= x^3 + 3/2 x^2y^2 + 3/4 xy^4 + y^6/8`
