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Turkish NMO First Round - 2012 Problem - 09 {Geometry}

Source:

July 1, 2012
geometry

Problem Statement

The chord [CD][CD] of the circle with diameter [AB][AB] is perpendicular to [AB][AB]. Let MM and NN be the midpoints of [BC][BC] and [AD][AD], respectively. If BC=6|BC|=6 and AD=23|AD|=2\sqrt{3}, then MN=?|MN|=?
<spanclass=latexbold>(A)</span> 4<spanclass=latexbold>(B)</span> 32<spanclass=latexbold>(C)</span> 21<spanclass=latexbold>(D)</span> 5<spanclass=latexbold>(E)</span> None <span class='latex-bold'>(A)</span>\ 4 \qquad <span class='latex-bold'>(B)</span>\ 3 \sqrt 2 \qquad <span class='latex-bold'>(C)</span>\ \sqrt{21} \qquad <span class='latex-bold'>(D)</span>\ 5 \qquad <span class='latex-bold'>(E)</span>\ \text{None}