Question

If the first term of an A.P. is 5, the last term is 15 and the sum of first n terms is 30, then find the value of n.

Answer 1

a = 5

T_n = l = 15

S_n = 30

n = ?

S_n = `n/2(a + l)`

⇒ 30 = `n/2(5 + 15)`

⇒ 60 = n × 20

⇒ 3 = n