`h)` `( x + 1 ) ( x - 4 ) ( x + 2 ) ( x - 8 ) + 4x^2`

`= [ ( x + 1 ) ( x - 8 ) ] [ ( x - 4 ) ( x + 2 ) ] + 4x^2`

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

Đặt `x^2 - 2x - 8 = t ( t > 0 )` ta được :

`( t - 5x )t + 4x^2`

`= t^2 - 5xt + 4x^2`

`= ( t - x ) ( t - 4x )`

`= ( x^2 - 2x - 8 - x ) ( x^2 - 2x - 8 - 4x )`

`= ( x^2 - 3x - 8 ) ( x^2 - 6x - 8 )`

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`i)` `x ( x + 1 ) ( x + 2 ) ( x + 3 ) + 1`

`= [ x ( x + 3 ) ] [ ( x + 1 ) ( x + 2 ) ] + 1`

`= ( x^2 + 3x  )( x^2 + 3x + 2 ) + 1`

Đặt `x^2 + 3x + 1 = t ( t > 0 )` ta được :

`( t - 1 ) ( t + 1 ) + 1`

`= t^2 - 1 + 1`

`= t^2`

`= ( x^2 + 3x + 1 )^2`

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`k)` Đặt `x^2 + x = t ( t > 0 )` ta được :

`t^2 - 2t - 15`

`= ( t^2 - 2t + 1 ) - 16`

`= ( t - 1 )^2 - 4^2`

`= ( t + 3 )( t - 5 )`

`= ( x^2 + x + 3 ) ( x^2 + x - 5 )`