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Sum of N Terms of an AP

Find the sum ...

Question

Find the sum of first $20$ terms of the series $1 + 2 + 3 +$ ........

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Solution

The given series is that of natural numbers. Hence, we can find the sum using the relation.
$S_{n} = \frac{n (n + 1)}{2}$ here, n = 20
$S_{20} = \frac{20 (20 + 1)}{2} = \frac{20 \times 21}{2}$
$S_{20} = 210$

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