∵f（x）=5x5+2x4+3.5x3-2.6x2+1.7x-0.8=（（（（5x+2）x+3.5）x-2.6）x+1.7）x-0.8∴v3=（（5x+2）x+3.5）x-2.6将x=5代入得v3=（（5×5+2）×5+3.5）×5-2.6=689.9故答案为689.9
