A geometric sequence has first term 5 and common ratio 3. Find the sum of its first 3 terms.
Answer and explanation
Correct answer: A. 65
- Step 1: Use S_n = a(r^n - 1)/(r - 1) for r not equal to 1.
- Step 2: S = 5(3^3 - 1)/(3 - 1) = 5 x 26 / 2 = 65.
Answer
65.
Why not the others: '45' gives the last term instead of the sum; '135' gives the (n+1)th term; '70' adds the first term twice.
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