Đáp án `+` Giải thích các bước giải:
` B1:`
` A = (2 + x)^2 - x(x + 3) `
`= 4 + 4x + x^2 - (x^2 + 3x) `
`= 4 + 4x + x^2 - x^2 - 3x `
` = 4 + (x^2 - x^2) + (4x - 3x) `
`= 4 + x `
` B = (1 -x)^2 + 2(x-1) + 1`
`= 1 - 2x + x^2 + 2x- 2 + 1`
`= ( 1 + 1 - 2) + (-2x + 2x ) + x^2`
`= x^2 `
Hay :
` = (1-x)^2 - 2.(1-x) . 1 + 1^2`
`= [(1-x) - 1]^2`
`= (1 - x - 1)^2`
`= x^2`
` C = (x-3y)(x^3 + 2xy + 9y^2) - x(x-y)(y+x) `
`= (x - 3y)[(x^2 + x.3y + (3y)^2] - x(x^2 - y^2) `
`= x^3 - (3y)^3 - x^3 + xy^2 `
`= xy^2 * 27y^3 `
` B2:`
` a) (x-5)^2 - (x+4)(x-4) = 20 `
`-> x^2 - 10x + 25 - x^2 + 16 = 20`
`-> -10x = -21`
`-> x =21/10 `
Vậy `x=21/10`
` b) (x+2)(x^2 - 2x + 4) - x(x^2 + 2) = 15`
`-> x^3 + 2^3 - x^3 - 2x = 15`
`-> -2x = 15 - 8 = 7`
`-> x = -7/2`
Vậy `x=-7/2`
