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The diagonal of a rectangle is the third and longest side of a triangle with sides the same as those of the rectangle, so its length is the square root of the sum of the squares of the lengths of the sides of the triangle, (Pythoagoras' Theorem) which are also the sides of the rectangle. If the rectangle is 3 inches by 4 inches, then the diagonal is the square root of 3 squared (= 9) and 4 squared (= 16) so the diagonal is the square root of 16 + 9 = 25, giving it the length of 5 inches.
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