`1) (2x+1)^3 `
`= (2x)^3 + 3 . (2x)^2 . 1 + 3 . 2x . 1^2 + 1^3 `
`= 8x^3 + 12x^2 + 6x + 1`
`2) (x-2)^3 `
`= x^3 - 3.x^2.2 + 3.x.2^2 - 2^3`
`= x^3 - 6x^2 + 12x - 8`
`3) (x+3)^3 `
`= x^3 + 3. x^2 . 3 + 3. x . 3^2 + 3^3 `
`= x^3 + 9x^2 + 27x + 27`
`4) (x-1/2)^3`
`= x^3 - 3 . x^2 . 1/2 + 3 . x . (1/2)^2 - (1/2)^3`
`= x^3 - 3/2x^2 + 3/4x - 1/8`
`5) (x+2)^3`
`= x^3 + 3.x^2.2 + 3.x.2^2 + 2^3`
`= x^3 + 6x^2 + 12x +8`
`6) (x-3)^2 `
`= x^2 - 2.x.3 + 3^2 `
`= x^2 - 6x + 9`
`7) (2x+3)^2 `
`= (2x)^2 + 2 . 2x . 3 + 3^2 `
`= 4x^2 + 12x + 9`
`8) (x+1/2)^3 `
`= x^3 + 3 . x^2 . 1/2 + 3 . x . (1/2)^2 + (1/2)^3`
`= x^3 + 3/2x^2 + 3/4x + 1/8`
`9) (2x+4)^3`
`= (2x)^3 + 3 . (2x)^2 . 4 + 3 . 2x . 4^2 + 4^3 `
`= 8x^3 + 48x^2 + 96x + 64`
`=>` Giải thích cách làm:
Áp dụng những hằng đẳng thức đáng nhớ sau :
`(a+b)^3 = a^3 + 3a^2b + 3ab^2 + b^3`
`(a-b)^3 = a^3 - 3a^2b + 3ab^2 - b^3`
`(a-b)^2 = a^2 - 2ab + b^2`
`(a+b)^2 = a^2 + 2ab+b^2`
