( x + y + z )” - ( x - y - z )” = [ (x+y+z) + (x-y-z) ][ (x+y+z) - (x-y-z) ] = ( x +y +z +x -y -z )( x +y +z -x +y +z ) = ( 2x )( 2y + 2z ) = 4x( y + z )
