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Question
Find the sum of first n natural numbers.
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Solution
The first n natural numbers are:
1, 2, 3, 4...
a = 1, d = 1, Total terms = n
\[S_n = \frac{n}{2}\left[ 2a + (n - 1)d \right]\]
\[ \Rightarrow S_n = \frac{n}{2}\left[ 2 \times 1 + (n - 1)1 \right]\]
\[ \Rightarrow S_n = \frac{n}{2}\left[ 2 + (n - 1)1 \right]\]
\[ \Rightarrow S_n = \frac{n}{2}\left[ n + 1 \right]\]
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