Dominoes on a mutilated chessboard
Statement
Two opposite corner squares are removed from an chessboard. Can the remaining squares be tiled by dominoes ( rectangles)?
Hint
Use the usual black and white colouring and count the squares of each colour.
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Solution
Idea. Every domino covers exactly one black and one white square.
Colouring. Colour the board like a chessboard. Two opposite corners, for instance and , have the same colour: and are both even.
Counting. The full board has black and white squares. Removing two squares of the same colour leaves of one colour and of the other.
Contradiction. Any tiling by dominoes covers exactly black and white squares. Impossible.
Answer. No.
Remark. If the two removed squares have different colours, a tiling always exists (Gomory's theorem).
Lesson. Colouring: choose a colouring in which every tile covers a prescribed number of squares of each colour, then compare with the colour counts of the region.