x" - 3x - 40
= x" - 8x + 5x - 40
= x( x - 8 ) + 5( x - 8 )
= ( x + 5 )( x - 8 )
或者
= x" + 5x - 8x - 40
= x( x + 5 ) - 8( x + 5 )
= ( x - 8 )( x + 5 )