Problem 34 of 34fun-034
A theater has 15 rows of seats. The first row has 20 seats, and each row after the first has 2 more seats than the row in front of it. How many seats are in the theater?
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Solution walkthrough1:48
- The rows hold seats: an arithmetic sequence that starts at and adds each row.
- Row is steps after row : seats.
- Add a flipped copy of the rows. Every row becomes seats, so the two copies hold .
- The theater is half the rectangle: .
- Traps: ignores the extra seats, takes steps to the last row, and forgets to halve.