设这个等比数列为 a1,a2,a3,,,,,,,a(n-2),a(n-1),an 可知 a1+a2+a3= 3a2 = 34 ;a2 = 34/3a(n-2)+a(n-1)+an = 3a(n-1) = 146 ; a(n-1) = 146/3 Sn = (a(n-1)+a2)*n/2 = 30n = 390 所以 n = 13
