Question

if for an A.P. t8 = 36, find s15

Answer 1

t8=36
=> a+7d=36......(1)

Now,
$s15 = \frac{15}{2}(2a + 14d) \\ = \frac{15}{2} \times 2(a + 7d) \\ = \frac{15}{2} \times 2 \times 36.......(using \: 1) \\ = 15 \times 36 \\ = 540$

Answer 2

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