当x=7时,代数式ax³+bx-8的值是8
343a+7b-8=8

当x=-7时,
a/2x³+b/2x+8
-343a/2-7b/2+8
=-1/2(343a+7b-8-8)
=-1/2×(8-8)
=-1/2×0
=0