Question

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

A. 87 

B. 88 

C. 89 

D. 90

Answer 1

Option : (C)

Here,

a₇ = 34 and a₁₃ = 64 

i.e.,

a + 6d = 34 ...(i) 

And,

a +12d = 64 ...(ii) 

Subtracting (i) from (ii) we get, 

(12-6) d = 64 - 34 

∴6d = 30 

∴ d = 5 

Substituting d = 5 in (i) we get, 

a+6(5) = 34 

∴a + 30 = 34 

∴ a = 4 

Now,

18^th term is given by a₁₈ = a+17d 

= 4+17×5 

= 89