Giải thích các bước giải:
`a) (x+3)(x+5)`
`=x(x+5)+3(x+5)`
`=x^2+5x+3x+15`
`=x^2+8x+15`
`b) (x-7)(x+4)`
`=x(x+4)-7(x+4)`
`=x^2+4x-7x-28`
`=x^2-3x-28`
`c) (x-1)(x+3)(x+7)`
`=[x(x+3)-1(x+3)](x+7)`
`(x^2+3x-x-3)(x+7)`
`=(x^2+2x-3)(x+7)`
`=x(x^2+2x-3)+7(x^2+2x-3)`
`=x^3+2x^2-3x+7x^2+14x-21`
`=x^3+(7x^2+2x^2)+(14x-3x)-21`
`=x^3+9x^2+11x-21`