首先求得当x=4-时,x2-8x+15=1,然后将原式化为x4-6x3-2x2+18x+23=x2(x2-8x+15)+2x(x2-8x+15)-(x2-8x+15)-20x+38,即可将原式化简,然后代入x=4-,即可求得答案.
【解析】
∵当x=4-时,x2-8x+15=(x-3)(x-5)=(1-)(-1-)=1,
∴
=x4-6x3-2x2+18x+23
=x2(x2-8x+15)+2x(x2-8x+15)-(x2-8x+15)-20x+38
=x2+2x-1-20x+38
=x2-18x+37
=(x2-8x+15)-10x+22
=1-10x+22
=23-10x,
∴当x=4-时,原式=23-10(4-)=10-17.
故答案为:10-17.