已知半径为5cm的圆O上弦长AB=8cm,则O到弦AB的距离为( )。
3cm
.Jack and I can go to the lecture ___
A.but neither can Mary
B.and do Mary can
C.but Mary can't
D.and Mary also can’t
等比数列{an}中,有a3a4a5a6a7a8=729.() (1)a5a6=9; (2)a4a5a6a7=81.