Find the sum of first $15$ terms of an A.P., whose nth term is $3 - 2 n$ .

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Solution

First term, $a = 1, 15^{t h}$ term $a_{15} = 3 - 30 = - 27$
Sum of 15 terms $= \frac{n}{2} (a + l (l a s t t e r m)) = \frac{15}{2} (1 - 27) = 15 (- 13) = - 195$

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