在等比数列an中,S4=65,q=2/3则a1=?

Question

在等比数列an中,S4=65,q=2/3则a1=?

Answer 1

因为成等比数列,所以S4=a1(1-(2/3)^4) / 1-2/3=65
a1(1-16/81) / 1/3=65
a1-16/81a1/ 1/3=65
(a1-16/81a1) * 3=65
3a1-16/27a1 =65
81/27a1-16/27a1=65
65/27a1=65 所以a1=27