If 7th and 13th terms of an A.P. be 34 and 64 respectively, then its 18th term is

Here, a7 = 34


a13 = 64


a7 = a + 6d = 34……….(1)


a13 = a + 12d = 64…………(2)


Subtracting (1) from (2)


6d = 30


D = 5


Multiplying (1) by 2


2a + 12d = 68………………….(3)


Subtracting (2) from (3)


a = 4


a18 = a + (n–1) d


a18 = 4 + (17) 5


a18 = 89 = C

1