Which term of the AP : 3, 8, 13, 18, . . . ,is 78?

: Given, a = 3, d = a₂ – a₁ = 8 – 3 = 5, a_n = 78, n = ?

We know that a_n = a + (n – 1)d

78 = 3 + (n – 1)5

 (n – 1)5 = 78 – 3 = 75

n – 1 = 15

n = 15 + 1 = 16

Thus, 78 is the 16th term of given AP.