X³+X²-9X-8=X(X²+X-9)-8 (1)
X²-2X-3=0,得X²-2X=3,得X²+X-X-2X=3 得X²+X=3X+3 (2)
X²-2X-3=0,得X(X-2)=3 (3)
(2)带入（1）得X(3X+3-9)-8 得3X(X-2) -8 (4)
(3)带入（4）得3×3-8=1 所以X³+X²-9X-8=1