I1.4 There are $c$ rows in a concert hall and each succeeding row has two more seats than the previous row. If the middle row has 64 seats, how many seats does the concert have?
There are altogether 31 rows. The $16^{\mathbb{1 1}}$ row is the middle row, which has 64 seats.
The $15^{\text {th }}$ row has $64-2=62$ seats.
The $14^{\text {th }}$ row has $64-2 \times 2=60$ seats.
$\qquad$
The $1^{\text {st }}$ row has $64-2 \times 15=34$ seats.
Total number of seats $=\frac{n}{2}[2 a+(n-1) d]=\frac{31}{2}[2 \cdot 34+(31-1) \cdot 2]=1984$
$$
\begin{aligned}
\text { Method } 2 \text { Total number of seats } & =\left(1^{\text {st }} \text { row }+31^{\text {st }} \text { row }\right)+\left(2^{\text {nd }} \text { row }+30^{\text {th }} \text { row }\right)+\ldots+16^{\text {th }} \text { row } \\
& =(64+64)+(64+64)+\ldots+64(31 \text { terms })=1984
\end{aligned}
$$