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Question
Find the sum of the following arithmetic progression :
50, 46, 42, ... to 10 terms
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Solution
50, 46, 42 ... to 10 terms
\[\text { We have }: \]
\[ a = 50, d = \left( 46 - 50 \right) = - 4\]
\[n = 10\]
\[ S_n = \frac{n}{2}\left[ 2a + (n - 1)d \right]\]
\[ = \frac{10}{2}\left[ 2 \times 50 + (10 - 1)( - 4) \right]\]
\[ = 5\left[ 100 - 36 \right] = 320\]
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