# How do you find the sum of the arithmetic sequence given a1 = 5, an = 3, n = 20?

Jun 24, 2015

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#### Explanation:

The formula for sum of an A.P when the first term $a 1$, number of terms $n$ and last term $a n$ are given is:

 color(blue)(Sum = (n/2)(a1+an)

$= \frac{20}{2} \left(5 + 3\right)$

$= 10 \times 8$

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