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If each term of a geometric progression a 1 , a 2 , a 3 , ….. with a 1 = 8 1 and a 2 ≠ a 1 , is arithmetic mean of the next two terms and S n = a 1 + a 2 + .... + a n , then S 20 – S 18 is equal to

1. –2 18
2. 2 18
3. –2 15
4. 2 15

Correct answer: –2 15

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