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ICSE Class 10 Answered

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Asked by Kanwaranita10 | 03 Mar, 2020, 02:44: PM
Expert Answer
Given that terms of a geometric progresion are t4 = 10, t6 = 40 and tn = 640
ar3 = 10 ...(i) and ar5 = 40 ...(ii)
Dividing (ii) by (i) we get r = ± 2
But it is given that ratio is positive hence, r = 2
We have ar3 = 10
8a = 10
a = 5/4
Also tn = 640
arn - 1 = 640
rn - 1 = 512 = 83 = 29
n - 1  = 9
n = 10
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