Geometric series : Find common ration 'r'

Question

Find common ration of a finite geometric series
If the first term is 11 and sum of first 12 terms is 2922920.

After applying the formula I got $265720 = (1 - r^{12})/(1-r)$ but I don't know how can I solve for it further.

Answer 1

I doubt whether an analytical solution like you probably expect is possible. What I would do instead is: $$265720 = \frac{r^{12} - 1}{r-1} $$ $$265720 \approx \frac{r^{12}}{r} = r^{11} $$ $$r \approx 3$$ And then verifying that $3$ actually satisfies the equation.