Câu `1`
`a)`
`3x^2 (2x^2 - 5x + 4) - (6x^4 - 15x^3)`
`= 6x^4 - 15x^3 + 12x^2 - 6x^4 + 15x^3`
` = (6x^4 - 6x^4) +(15x^3-15x^3) + 12x^2`
` = 12x^2`
`b)`
`-x^2 (-2x^2 + 3x - 1) - (7x^4 - 5x^3)`
` = 2x^4 - 3x^3 + x^2 - 7x^4 + 5x^3`
` = (2x^4 - 7x^4) +(5x^3-3x^3) +x^2`
` = -5x^4 + 2x^3+x^2`
`c)`
`-2x^2 (3x - 1/4 - 2x^3) - (4x^5 - 6x^3)`
` =-6x^3 + 1/2x^2 + 4x^5 - 4x^5 + 6x^3`
` = (4x^5 - 4x^5) + (6x^3 - 6x^3) + 1/2 x^2`
` = 1/2 x^2`
`d)`
`(-3x+2)(3x-4) + (3x-1)(3x+1)`
` = -9x^2 + 12x + 6x - 8 + 9x^2 -1`
` = (9x^2 - 9x^2) +(12x+6x) - (8+1)`
` = 18x -9`
Câu `2`
`a)`
`(2x+1)(x-5) - x (3+2x) = 26`
`=> 2x^2 - 10x + x - 5 - 3x - 2x^2 = 26`
`=> (2x^2 - 2x^2) + (x - 3x -10x) = 26 + 5`
`=> -12x = 31`
`=> x = -31/12`
Vậy `x=-31/12`
`b)`
`(7-4x)(4x+7)-8x(11-2x) = -39`
`=> 49 - 16x^2 - 88x + 16x^2 = -39`
`=> (16x^2-16x^2) - 88x = -39 - 49`
`=> -88x = -88`
`=>x=1`
Vậy `x=1`
