Art of Problem Solving

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1975 AHSME Problems/Problem 30

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Problem 30

Let $x=\cos 36^{\circ} - \cos 72^{\circ}$ . Then $x$ equals

$\textbf{(A)}\ \frac{1}{3}\qquad \textbf{(B)}\ \frac{1}{2} \qquad \textbf{(C)}\ 3-\sqrt{6} \qquad \textbf{(D)}\ 2\sqrt{3}-3\qquad \textbf{(E)}\ \text{none of these}$

Solution

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