Sequences and Series (003)

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

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