Which term of the series 5, 10, 20, 40, ….. is 1280?
10th
9th
8th
None of these
Explanation:
Clearly, 5 x 2 = 10, 10 x 2 = 20, 20 x 2 = 40,…..So, the series is a G.P. in which a = 5 and r = 2.Let 1280 be the rath term of the series.Then, 5×2n-1 = 1280 2n-1 = 256 = 28 n – 1 = 8 n = 9.
Clearly, 5 x 2 = 10, 10 x 2 = 20, 20 x 2 = 40,…..So, the series is a G.P. in which a = 5 and r = 2.Let 1280 be the rath term of the series.Then, 5×2n-1 = 1280 2n-1 = 256 = 28 n – 1 = 8 n = 9.