The diagonal of length 2r forms the hypotenuse of a right angled triangle with one leg ( the width of the rectangle) = x. So by Pythagoras:-
(2r)^2 = h^2 + x^2 where h = height of the rectangle
h^2 = 4r^2 - x^2
h = √ (4r^2 - x^2) answer
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