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  • Brainly User
2015-09-02T02:41:35+08:00

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I assume that you are looking for the first term (a1) given the sum and ratio of the first five terms of a geometric sequence

Formula:  Sn = a1 (1-r^n)
                            1-r 

n = 5;          S5= 31/4;      r (ratio) = 1/2;        a1=unknown  

31/4 = a1 (1-(1/2)^5)
                1-1/2

31/4 = a1 (1-1/32) 
               1/2

31/4 = a1 (31/32) 
                 1/2

(1/2)(31/4) = a1 (31/32)

31/8 = a1 (31/32)

(32/31)(31/8) = a1 (31/32)(31/31)

4 = a1  

FINAL ANSWER:  a1 = 4 (the first term of the first five terms of a geometric sequence given their sum and ratio)
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