help me giúp mk câu 4 thôi !

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`4,`

`(x+1)(x+3)(x+5)(x+7)+2002`

`= (x^2+7x +x+7)(x^2+5x+3x+15) + 2002`

`= (x^2+8x+7)(x^2+8x+15)+2002`

Đặt `x^2+8x+7=t`

`= t (t+8)+2002`

`=t^2+8t + 16 + 1986`

`=(t+4)^2+1986`

`= (x^2+8x+7+4)^2+1986`

`= (x^2+8x+13)^2 + 1986` 

`= (x^2+8x+13)^2 - 1^2 + 1987`

`= (x^2+8x+13-1)(x^2+8x+13+1) + 1987`

`= (x^2+8x+12)(x^2+8x+14)+1987`

Vì `(x^2+8x+12)\vdots (x^2+8x+12)`

`=>(x^2+8x+12)(x^2+8x+14)\vdots (x^2+8x+12)`

`=> (x^2+8x+12)(x^2+8x+14)+1987\vdots (x^2+8x+12)` dư `1987`

`=>(x+1)(x+3)(x+5)(x+7)+2002 : (x^2+8x+12)` dư `1987`