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A person has 2 parents, 4 grandparents, ...

A person has 2 parents, 4 grandparents, 8 great grandparents, and so on. Find the number of his ancestors during the ten generations preceding his own.

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Here `a=2, r=2` and `n=10`
Using the sum formula `S_n=a(r^n-1)/r-1`
We have `S_10=2(2^10-1)=2046`
Hence, the number of ancestors preceding the person is `2046 .`
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