Question

Which term of the A.P. 3, 8, 13, 18, ..., is 78?

Answer 1

3, 8, 13, 18, …

For this A.P.,

a = 3

d = a₂ − a₁

= 8 − 3

= 5

Let n^th term of this A.P. be 78.

a_n = a + (n − 1) d

78 = 3 + (n − 1) 5

75 = (n − 1) 5

(n − 1) = 15

n = 16

Hence, 16^th term of this A.P. is 78.