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Question
Find the sum of the following arithmetic progression :
41, 36, 31, ... to 12 terms
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Solution
41, 36, 31 ... to 12 terms
\[\text { We have: }\]
\[ a = 41, d = \left( 36 - 41 \right) = - 5\]
\[n = 12\]
\[ S_n = \frac{n}{2}\left[ 2a + (n - 1)d \right]\]
\[ = \frac{12}{2}\left[ 2 \times 41 + (12 - 1)( - 5) \right]\]
\[ = 6 \times 27 = 162\]
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