2X³-3X²-11X+8
=2X³-6X²-2X+3X²-9X+8
=2X(X²-3X-1)+3X²-9X-3+3+8
=2X(X²-3X-1)+3(X²-3X-1)+11
因为X²-3X-1=0
所以2X(X²-3X-1)+3(X²-3X-1)+11=11
2X³-3X²-11X+8
=2X³-6X²-2X+3X²-9X+8
=2X(X²-3X-1)+3X²-9X-3+3+8
=2X(X²-3X-1)+3(X²-3X-1)+11
因为X²-3X-1=0
所以2X(X²-3X-1)+3(X²-3X-1)+11=11