Question

Find the sum of the first 20 terms of the geometric series -2 + 4 + -8 + ....

Answer 1

Here is the formula.
\[S_{n} = \frac{a_{1}(1 - r^{n})}{1 - r}\]where \[a_{1} = -2 \space and \space n = 20 \space and \space r = -2\]I recommend using a calculator :)