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