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Difficulty: 3/102022 NMTC 2022 (QII-35)

The number of rectangles that chessboard has...

Guide / Hint

Hint 1: Note that a rectangle on a grid is defined by selecting 2 horizontal grid lines and 2 vertical grid lines.

Hint 2: For an chessboard, there are 9 horizontal lines and 9 vertical lines. Choose 2 of each: .

Hint 3: Compute to find the total rectangles.

Solution

Step 1 (Analyze Chessboard Grid): An chessboard consists of 8 rows and 8 columns of squares. To form this grid, there are exactly:

  • 9 horizontal lines

  • 9 vertical lines

Step 2 (Rectangle Construction): A rectangle is uniquely formed by choosing exactly 2 distinct horizontal lines (which define the top and bottom boundaries) and 2 distinct vertical lines (which define the left and right boundaries).

Step 3 (Calculate Combinations): The number of ways to choose these lines is:

  • Ways to choose 2 horizontal lines from 9:

  • Ways to choose 2 vertical lines from 9:

Using the product rule, the total number of rectangles is:

Step 4 (Conclusion): The chessboard contains exactly 1296 rectangles.

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