# Geometric series - Check my answer

What is the first term in a geometric series with ten terms, a common ratio of 0.5 and a sum of 511.5?

I got 256 as my answer.

511.5=a1[1-(.5)^10]/1-1/2
511.5=a1(1023/1024)/1/2
511.5=a1(1023/512) ----> divide by both sides
256=a1

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3. 👁 191
1. Looks good to me.

You can also check by adding
256 + 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 + 0.5

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posted by drwls
2. The first term of a geometric series is 7 and the ratio between terms is 0.5. Find S4.

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posted by Cheryl

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