Question

If 18^th and 11^th term of an A.P. are in the ratio 3 : 2, then its 21^st and 5^th terms are in the ratio

(a) 3 : 2

(b) 3 : 1

(c) 1 : 3

(d) 2 : 3

Answer 1

In the given problem, we are given an A.P whose 18^th and 11^th term are in the ratio 3:2

We need to find the ratio of its 21^st and 5^th terms

Now, using the formula

,

Where,

a = first tem of the A.P

n = number of terms

d = common difference of the A.P

So,

Also,

Thus,

Further solving for a, we get

Now,

Also,

So,

Using (1) in the above equation, we get

Thus, the ratio of the 21^st and 5^th term is

Therefore the correct option is (b).