## Problem 1

An amphitheater-shaped concert hall has 30 rows, and each row has 3 more seats than the row in front of it. Calculate how many seats the hall has, knowing that the last row has 117 seats.
## Problem 1

![](https://cdn.mathpix.com/cropped/2024_06_07_d20c4d07f02af3edba72g-2.jpg?height=66&width=1661&top_left_y=688&top_left_x=243)

![](https://cdn.mathpix.com/cropped/2024_06_07_d20c4d07f02af3edba72g-2.jpg?height=62&width=1667&top_left_y=777&top_left_x=240)

$$
\begin{aligned}
& S_{30}=\frac{\left(2 \cdot a_{1}+29 r\right) \cdot 30}{2}=2205 \text { seats. }
\end{aligned}
$$