c^2 + c^2d - c - d = c( c - 1) +d(c^2 - 1 ) = c ( c - 1 ) + d (c - 1 )( c + 1 ) = ( c - 1 )( c + d( c + 1)) = ( c - 1 )( c + cd + d )
C(c+d)-(c+d)=(c+d)(c-1)
