Let a, ar, ar2, be an infinite G.P. If n=0 arn = 57 and n=0 a3r3n = 9747, then a + 18r is equal to
- 27
- 38
- 31
- 46
Answer
(3) 31
Solution
Both are infinite G.P.s (so |r| < 1): a1 - r = 57 …(i) and a31 - r3 = 9747 …(ii)
Cubing (i) and dividing by (ii): 1 - r3(1 - r)3 = 5739747 = 19
1 + r + r2(1 - r)2 = 19
18r2 - 39r + 18 = 0 6r2 - 13r + 6 = 0
r = 23 or 32; since |r| < 1, r = 23
So from (i), a = 57 (1 - 23 ) = 19, hence a + 18r = 19 + 12 = 31
Answer: (3)