58. As shown in the figure below, a triangle has 2 sides each of length 5 feet and a 3rd side of length x feet. The degree measure of the angle between the 2 sides that are 5 feet long is θ. In terms of x, cos θ = ?

(Note: For any triangle, if a, b, and c are the lengths of the sides opposite ∠A, ∠B, and ∠C, respectively, then a² = b² +c² - 2bc cos ∠A.)

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