Find the sum of the first 20 terms in an arithmetic progression whose first term is 7 and last term is 117.
A
2480
B
1240
C
620
D
124
Verified Correct Answer: B.
Step-by-Step Mathematical Solution
Given the first and last term of an A.P, the sum of the terms is given by
\(S_{n} = \frac{n}{2} [a + l]\)
where a = first term; l = last term and n = number of terms.
\(\therefore S_{20} = \frac{20}{2} [7 + 117]\)
= \(10 (124)\)
= 1240
Source: Myschool Past Questions Archive (Record #31416)