Question

Find the um of first 15 terms

Answer 1

$\mathfrak{here \: is \: your \: answer} \\ s_{15} = \frac{n}{2} ({2a + (n - 1)d}) \\ s_{15} = \frac{15}{2} (2a + (15 - 1)d) \\ s_{15} = \frac{15}{2} (2a + 14d) \\ s_{15} = \frac{15}{2} (2(a + 7d)) \\ s_{15} = 15(a + 7d) \\ s_{15} = 15a + 105d$

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