凸n边形有f(n)条对角线,则凸n+1边形有f(n+1)条对角线数为( )
等差数列中,已知公差,且,则等于( )
已知由正数组成的等比数列{an}中,公比q="2," a1·a2·a3·…·a30=245, 则a1·a4·a7·…·a28= ( )
如果数列是等差数列,则( )
已知等差数列的公差为2,若成等比数列, 则=( )
在各项都为正数的等比数列{an}中,首项a1=3,前三项和为21,则a3+ a4+ a5="(" )