Question

(｀・ω・´)

求這題＞＜

若一等比數列<an>共有 12 項,且al+a2+a3-10, a4+a5+a6-15,則此數列所有項的和為
( 325/4 1.
0

Answer 1

✨ 最佳解答 ✨

呵

a1+a2+a3
=a1+a1×r+a1×r^2
=a1(1+r+r^2) = 10

a1(r^3+r^4+r^5)=15 = a1×r^3(1+r+r^2)

所以r^3=3/2

所求
=10+15+45/2+135/4
=(40+60+90+135)/4
=325/4