(1)Sn＝a1(1－q^n)/(1－q)∴9/8×(1－q^n)/(1－2/3)＝65/24 ∴q^n＝(2/3)^n＝16/81＝(2/3)^4∴n＝4(2)∵a、b、c三个数成等差数列,且a、b、c+3和a+1、b、c成等比数列∴2b＝a＋c①b²＝a(c＋3)② ...