If the sum of the second, fourth and sixth terms of a G.P. of positive terms is 21 and the sum of its eighth, tenth and twelfth terms is 15309, then the sum of its first nine terms is :
- 757
- 755
- 750
- 760
Answer
(1) 757
Solution
Let first term = a, common ratio = r (a, r > 0).
ar + ar3 + ar5 = ar(1 + r2 + r4) = 21 ...(i)
ar7 + ar9 + ar11 = ar7(1 + r2 + r4) = 15309 ...(ii)
(ii) ÷ (i): r6 = 729 r = 3
From (i): 3a(1 + 9 + 81) = 21 273a = 21 a = 113
S9 = a(r9 - 1)r - 1 = 113 19683 - 12 = 1968226 = 757
Answer: (1)