Geometric Sum 15Da09
1. **State the problem:** Find the sum of the first 60 terms of the geometric sequence \(\frac{1}{3}, \frac{1}{9}, \frac{1}{27}, \ldots\).
2. **Identify the first term and common ratio:**
- First term \(a = \frac{1}{3}\)
- Common ratio \(r = \frac{1}{9} \div \frac{1}{3} = \frac{1}{3}\)
3. **Formula for the sum of the first \(n\) terms of a geometric sequence:**
$$ S_n = a \frac{1-r^n}{1-r} $$
where \(a\) is the first term, \(r\) is the common ratio, and \(n\) is the number of terms.
4. **Apply the formula:**
$$ S_{60} = \frac{1}{3} \times \frac{1 - \left(\frac{1}{3}\right)^{60}}{1 - \frac{1}{3}} $$
5. **Simplify the denominator:**
$$ 1 - \frac{1}{3} = \frac{2}{3} $$
6. **Rewrite the sum:**
$$ S_{60} = \frac{1}{3} \times \frac{1 - \left(\frac{1}{3}\right)^{60}}{\frac{2}{3}} = \frac{1}{3} \times \frac{3}{2} \left(1 - \left(\frac{1}{3}\right)^{60}\right) $$
7. **Simplify:**
$$ S_{60} = \frac{1}{2} \left(1 - \left(\frac{1}{3}\right)^{60}\right) $$
8. **Interpretation:** Since \(\left(\frac{1}{3}\right)^{60}\) is an extremely small positive number, the sum is very close to \(\frac{1}{2}\).
**Final answer:**
$$ S_{60} = \frac{1}{2} \left(1 - \left(\frac{1}{3}\right)^{60}\right) $$