若x=-4是关于x的方程ax2-6x-8=0的一个解，则a=______．
把-4代入方程有：
16a+24-8=0
解得：a=-1．
故答案是：-1．
在距地面高15米处以10米/秒的速度竖直上抛一物体，从开始上抛到落地这段时间内物体的平均速度为______米/秒．（g=10m/s2）
The new stadium being built for the next Asian Games will be________the present one.[ ]
A. as three times big as
B. three times as big as
C. as big as three times
D. as big three times as
