Question

How many terms are there in the AP 7, 11, 15, ..., 139?

Answer 1

Here, a = 7, d = (11 − 7) = 4 and l = 139
Let there are n terms in the given AP.
Then T_n = 139
⇒ a + (n − 1)d = 139
​⇒ 7 + (n − 1) ⨯ ​4 = 139
⇒ 4n = 136
⇒ n = 34
Hence, there are 34 terms in the AP.