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A boy arranges exercise books in piles. The first pile has 2 exercise books. The second pile has 5 exercise books. The third pile has 8 exercise books. The eighth pile has 22 exercise books. If the boy continues arrangig exercise books this way, calculate the total number of exercise books used if he has arranged 25 piles.
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This is a real life number pattern with terms, 2,5,8, ...,22,....25

a=2

d=3, arithmetic sequence

We have to find the sum of books given 25 as the number of  terms(piles)

Sn  = n/2 x [2a+(n-1)d]

= 25/2 x [2x2+(25-1)3]

= 950 books
by Diamond (44,680 points)

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