等比数列：a1+a3=20,a2+a4=60 则a7+a8=?_数学解答

等比数列：a1+a3=20,a2+a4=60 则a7+a8=?

人气：385 ℃　时间：2019-11-09 04:19:44

解答

a2+a4 = a1*q+a3*q = 60
a1+a3 = 20
所以q = 3
a1+a3 = a1+a1*q*q = 20
所以a1 = 2
a7 =2*3^6
a8 = 2*3^7
a7+a8 = 5832