你为什么不穿一件漂亮的裙子呢?
A:Why don't you to wear a beautiful dress?
B:Why don't you wear a beautiful dress?
——________ you tell me how to write it in English?——I'm sorry, I can't.
某物体由静止开始,做加速度为a1的匀加速直线运动,运动时间为t1,接着物体又做加速度为a2的匀减速直线运动,经过时间t2,其速度变为零,则物体在全部时间内的平均速度为( )