To find the sum of an arithmetic series, use the sequence sum formula: \[ S = \frac{n}{2} (a + l) \]where:
- \( S \) is the sum of the series.
- \( n \) is the number of terms.
- \( a \) is the first term.
- \( l \) is the last term.
In our step-by-step solution, we identified the sequence first, then determined the number of terms as 21. Finally, we used the formula:
- \( S = \frac{21}{2} * (50 + 70) \)
- Simplify the sum inside the parentheses first: \( 50 + 70 = 120 \)
This led us to:
- \( S = 10.5 * 120 \)
- \( S = 1,260 \)
So, the sum of numbers from 50 to 70 is 1,260.