Question

Sequences and Series (003)

Answer 1

Solution:∵geometric progreesion formula is S=a(1-q^n)/1-q. a is first term,q is

common ratio, n is term.

But in this question"its sum to infinity is 15",

So （ 见图片 ）

∴ S=a/(1-q)

15=10/(1-q)

So q=-1/2

所以a3=aq^2

=10×（-1、2）^2

5/2