A geometric sequence has first term 2 and common ratio 4. Find the sum of its first 4 terms.
Answer and explanation
Correct answer: D. 170
- Step 1: Use S_n = a(r^n - 1)/(r - 1) for r not equal to 1.
- Step 2: S = 2(4^4 - 1)/(4 - 1) = 2 x 255 / 3 = 170.
Answer
170.
Why not the others: '128' gives the last term instead of the sum; '512' gives the (n+1)th term; '172' adds the first term twice.
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