If r = 3 and the sum of 6 terms of the geometric series is 3640, what is the first term?

sum formula = s = a(1-r^n) / (1-r)

3640 = a (1 - 3^6) / (1-3)

3640 = a (1-729) / (-2)

Cross multiply

3640 * (-2) = a * (1-729)

- 7280 = a * (-728)

7280 = a * 728

Dividing by 728

a = 10 : Answer

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