Question

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

A. 87 

B. 88 

C. 89 

D. 90

Answer 1

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