如果x2+2x=3，那么x4+7x3+8x2-13x+15=_．
问题描述：
如果x2+2x=3，那么x4+7x3+8x2-13x+15=______．
答
∵x2+2x=3
∴x4+7x3+8x2-13x+15=x2（x2+2x）+5x3+8x2-13x+15
=x2×3+5x3+8x2-13x+15
=5x3+11x2-13x+15
=5x（x2+2x）+x2-13x+15
=15x+x2-13x+15
=x2+2x+15
=3+15
=18
故答案为18