The sums of n terms of two arithmetic progressions are in the ratio 5n + 4 : 9n + 6. Find the ratio of their 18th terms.
Let a1, a2, and d1, d2 be the first terms and the common difference of the first and second arithmetic progression respectively.
Given, 
⇒ 
Putting n = 35 in the above equation
⇒ 
⇒ 
∴ 
Thus, the ratio of 18th term of both the A.P.s is 179: 321
9