Geometric progression

In geometric progression, a1 = 7, q = 5. Find the condition for n to sum first n members is sn≤217.

Correct answer:

n =  3

Step-by-step explanation:

a1=7 q=5 s=217  s=aq q1qn1 s/a1 (q1)=qn1 s/a1 (q1)+1=qn lns/a1 (q1)+1=nlnq  n=lnqln(s/a1 (q1)+1)=ln5ln(217/7 (51)+1)=3



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