數學題 幫我計計

(5-2x)(x+4)
= 5x+20-2x^2-8x
= -3x+20-2x^2
= -2x^2+20-3x <---------------(你鍾意有就有, 冇就冇, 只是方便我計)

(5-2x)(x+4)-(2x+7)
= -2x^2+20-3x-(2x+7)
= -2x^2+20-3x-2x-7
= -2x^2+13-5x

2007-02-20 19:07:24 補充：
上面個個計錯左, (5-2x)(x+4)-(2x+7)佢睇左(5-2x)(x+4)-(2x-7)

(5-2x)(x+4)
=(5-2x)(x)+(5-2x)(4)
=5x-2x(2次方)+20-8x
=5x-8x-2x(2次方)+20
=-3x-2x(2次方)+20

(5-2x)(x+4)-(2x+7)
=(5-2x)(x)+(5-2x)(+4)-(5-2x)(2x)+(5-2x)(7)
=5x-2x(2次方)+(20)-(8)-(10x)-(4x)+(35)-(14x)

(5-2x)(x+4)
=5x+20-2x^2-8x
=20-3x-2x^2

(5-2x)(x+4)-(2x-7)
=20-3x-2x^2-2x+7
=27-5x-2x^2