Question

If in an A.P., first term is 20, common difference is 2 and n^th term is 42, then find sum up to n terms.

(a) 12

(b) 42

(c) 352

(d) 372

I got this question in my homework.

The question is from Arithmetic Progression(A.P.) topic in portion Sequences and Series of Mathematics – Class 11

Answer 1

Correct option is (d) 372

To explain: We know, a=20, d=2, an=42.

a+(n-1) d = 42 => 20 + 2(n-1) = 42

=>2(n-1) = 42-20=22 => n-1 = 11 => n=12.

Sn = \(\frac{n}{2}\) (a+l) => S12 = \(\frac{12}{2}\) (20+42) = 6*62 = 372.