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Question
Find the sum of the following arithmetic progression :
3, 9/2, 6, 15/2, ... to 25 terms
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Solution
3, 9/2, 6, 15/2 ... to 25 terms
\[\text { We have }: \]
\[ a = 3, d = \left( 9/2 - 3 \right) = 3/2\]
\[n = 25\]
\[ S_n = \frac{n}{2}\left[ 2a + (n - 1)d \right]\]
\[ = \frac{25}{2}\left[ 2 \times 3 + (25 - 1)(3/2) \right]\]
\[ = \frac{25}{2} \times 42\]
\[ = 525\]
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