Question

13th term of an arithmatic sequence is 40. a. What is the sum of its 6th term and 20th term. b. What is the sum of its first 25 terms.​

Answer 1

Answer:

13th term of an arithmatic sequence is 40.

$a13 = a + 12d = 40$

a. What is the sum of its 6th term and 20th term.

$s = \frac{a1 + 5d+ a1 + 19d}{2} \\ s = \frac{2a1 + 24d}{2} \\ s = a1 + 12d$

b. What is the sum of its first 25 terms.

$s = \frac{a1 + 24d}{2}$