It is required to compare the diagonal of the rectangle and its side.
When you draw a diagonal of a rectangle, you divide it into two equal right-angled triangles, where the legs are the sides of the rectangle and the diagonal is the hypotenuse. And here the Pythagorean rule is turned on, where the square of the diagonal is equal to the sum of the squares of the sides of the rectangle.
Now imagine that one of the sides is larger than the diagonal. Then the square of the side will also be larger than the square of the diagonal, which cannot be at all, since the square of the other side of the rectangle will be a negative number.
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