Proof by Pigeonhole Principle
A standard chessboard has 64 squares, 32 white and 32 black. Each opposite (aka diagonal) corner has the same color. If you remove two corners, then the ratio of square colors becomes either 30 white and 32 black squares or 32 white and 30 black squares.
Note, each domino tiles two squares at once: 1 white square and 1 black square. Since the ratio of square colors is not 1:1 after removing the corners, it is impossible to cover the resulting chessboard without overlap or overhangs. By the Pigeonhole Principle, if we tile the color with less squares, there will only be 2 squares of the other color remaining -> either requiring an overlap or overhang.
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