How many terms of the series 3, -6, +12, – 24, + ….. are needed to make a total of 1 – 2\(^8\)?
- 12
- 10
- 9
-
8
The correct answer is: D
Explanation
3, - 6, +12, -24
a = 3, r = -2
\(Sn = \frac{a(1-r^n)}{1-r}\\
∴1-2^8 = \frac{3(1-(-2^{n}))}{1-(-2)}\\
1-2^8 = \frac{3(1-(-2^{n}))}{3}\)
1 - 2\(^8\) = 1 - (-2)\(^n\)
Since 1 - 2\(^8\) = 1 - (-2)\(^n\) ...... none