## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

The sequence is geometric with $r=\dfrac{5}{4}$. and $S_{50} \approx 350,319.62$
We will check whether the sequence is arithmetic or geometric. (1) A sequence is arithmetic if there exists a common difference $d$ among consecutive terms such that: $d=a_n−a_{n−1}$ The sum of the first n terms of an arithmetic sequence can be computed as: $S_n=\dfrac{n}{2}[2a_1+(n−1)d] (1)$ (2) A sequence is geometric if there exists a common ratio $r$ among consecutive terms such that: $r=\dfrac{a_n}{a_{n−1}}$ The sum of the first n terms of a geometric sequence can be computed as: $S_n= \dfrac{a_1(1-r^n)}{1-r} (2)$ where, $a_1 =\ First \ term$, $a_n$ = $n$th term, and $n =\ Number \ of \ Terms$ Substitute $n=1,2,3$ to list the first three terms: $a_1 =(\dfrac{5}{4})^1=\dfrac{5}{4} \\ a_2 = (\dfrac{5}{4})^2=\dfrac{25}{4} \\ a_3 = (\dfrac{5}{4})^3=\dfrac{125}{64}$ We see that there is no common difference, so the sequence is not arithmetic. Solve for the ratio of pairs of consecutive terms to obtain: $\dfrac{a_2}{a_1} = \dfrac{\frac{25}{16}}{\frac{5}{4}} = \dfrac{5}{4}$ and $\dfrac{a_3}{a_2} = \dfrac{\frac{125}{64}}{\frac{25}{16}}=\dfrac{5}{4}$ We see that the sequence has a common ratio of $r=\dfrac{5}{4}$. So, the sequence is geometric with $r=\dfrac{5}{4}$. In order to find the sum of the first 50 terms, we will substitute $a_1=\dfrac{5}{4}$ and $r=\dfrac{5}{4}$ into the formula in (2) to obtain: $S_{50} = \dfrac{5}{4} \left(\dfrac{1-\cdot(\frac{5}{4})^{50}}{1-\frac{5}{4}}\right) \approx 350,319.62$ Therefore, the sum of first 50 terms is equal to: $S_{50} \approx 350,319.62$