Đáp án:
`1, 2x(x^2+3x-5)-x(2x^2+1)`
`= 2x^3+6x^2-10x-2x^3-x`
`= (2x^3-2x^3)+6x^2-(10x+x)`
`= 6x^2-11x`
`2, (x-5)(x+5)-x(x+1)`
`= x^2-25-x^2-x`
`= (x^2-x^2)-x-25`
`= -x-25`
`3, (x+2)(x-4)-x^2`
`= x^2-4x+2x-8-x^2`
`= (x^2-x^2)-(4x-2x)-8`
`= -2x-8`
`4, (x+7)^2-x^2`
`= x^2+14x+49-x^2`
`= 14x+49`
`5, (15xy+5x^2y-10xy^2):(-5xy)`
`= (-5xy)(-3-x+2y):(-5xy)`
`= 2y-x-3`