A:he happened
B:did happen to him
C:did he happen
D:happened to him
一质点绕半径为R的圆匀速运动了半周,所用时间为t.在此过程中该质点的( )
已知⊙O的半径为15,弦AB的长为18,点P在弦AB上且OP=13,则AP的长为( )