Accumulation pattern and growth

This forms an arithmetic progression:

Sum=N/2[2F + (N -1)*D]

Sum =50/2[2*20 + (50 -1)*2]

Sum =25[40 + 98]

Sum =25 x 138

Sum =3,450 - number of seats in the theater.

Another way:

$\sum_{1}^{50}$ 20 + 2(n-1) = 3450 seats

Find the number of theater seats in an auditorium with 50 rows where
the first row has 20 seats and
each successive row has 2 more seats than the previous?

Formula:

$\begin{array}{|rcll|} \hline t_n &=& t_1 + (n-1)\cdot d \\ s_n &=& \left(\frac{t_1+t_n}{2} \right) \cdot n \\ \hline \end{array}$

$\begin{array}{|rcll|} \hline n &=& 50 \\ t_1 &=& 20 \\ d &=& 2 \\\\ t_{50} &=& 20 + (50-1)\cdot 2 \\ t_{50} &=& 20 + 49\cdot 2 \\ t_{50} &=& 118 \\\\ s_{50} &=& \left(\frac{20+118}{2} \right) \cdot 50 \\ s_{50} &=& \left(\frac{138}{2} \right) \cdot 50 \\ s_{50} &=&69 \cdot 50 \\ s_{50} &=&3450 \\ \hline \end{array}$

The number of theater seats in an auditorium is 3450