Posted by **Anonymous** on Tuesday, November 3, 2009 at 1:15am.

An auditorium has 49 rows of seats. The first row contains 40 seats. As you move to the rear of the auditorium, each row has 3 more seats than the previous row.

-How many seats are in the auditorium?

- math -
**Reiny**, Tuesday, November 3, 2009 at 7:31am
This is an arithmetic series

Sum(n) = (n/2)[2a + (n-1)d]

where a is the first term

n is the number of terms

d is the common difference, and

Sum(n) is the sum of the n terms

So you want Sum(49)

with a=40, d=-3 and n = 49

substitute

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