5、2B.16C.20D.2412.已知等差数列满足,则有() A. B. C. D.13.设是公差为正数的等差数列,若,,则()A.B.C.D.14.等差数列中,,则=()A.3B.4C.6D.1215.若一等差数列前四项的和为124,后四项的和为156,又各项的和为350,则此数列共有()A.10项B.11项C.12项D.13项16.等比数列中,,,则()A.12B.10C.8D.17.等差数列中,,,则()A.15B.25C.35D.4518.已知等比数列前10项的和为10,前20项的和为30,那么前30项的和为()A.60B.70C.90D.1263绵阳