Question

Find the sum of an arithmetic sequence if the first term is 3 and the last term is 39?

Answer 1

Step-by-step explanation:

$a = 3 - - - (1)$

$a + (n - 1)d = 39$

$let \: \: l \: \: be \: the \: last \: term$

$sn = \frac{n}{2} (a + l)$

$sn = \frac{n}{2} (3 + 39)$

$\: \: = \frac{n}{2} \times 42$

$sn = 21n$