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Geometry Level 5

In R E D \triangle RED , D R E = 7 5 \measuredangle DRE=75^{\circ} and R E D = 4 5 \measuredangle RED=45^{\circ} . R D = 1 |RD|=1 . Let M M be the midpoint of segment R D \overline{RD} . Point C C lies on side E D \overline{ED} such that R C E M \overline{RC}\perp\overline{EM} . Extend segment D E \overline{DE} through E E to point A A such that C A = A R CA=AR . Then A E = a b c AE=\frac{a-\sqrt{b}}{c} , where a a and c c are relatively prime positive integers, and b b is a positive integer. Find a + b + c a+b+c .


The answer is 56.

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1 solution

Johanz Piedad
Sep 13, 2015

Let P P be the foot of the perpendicular from A A to C R \overline{CR} , so A P E M \overline{AP}\parallel\overline{EM} . Since triangle A R C ARC is isosceles, P P is the midpoint of C R \overline{CR} , and P M C D \overline{PM}\parallel\overline{CD} . Thus, A P M E APME is a parallelogram and A E = P M = C D 2 AE = PM = \frac{CD}{2} . We can then use coordinates. Let O O be the foot of altitude R O RO and set O O as the origin. Now we notice special right triangles! In particular, D O = 1 2 DO = \frac{1}{2} and E O = R O = 3 2 EO = RO = \frac{\sqrt{3}}{2} , so D ( 1 2 , 0 ) D(\frac{1}{2}, 0) , E ( 3 2 , 0 ) E(-\frac{\sqrt{3}}{2}, 0) , and R ( 0 , 3 2 ) . R(0, \frac{\sqrt{3}}{2}). M = M = midpoint ( D , R ) = ( 1 4 , 3 4 ) (D, R) = (\frac{1}{4}, \frac{\sqrt{3}}{4}) and the slope of M E = 3 4 1 4 + 3 2 = 3 1 + 2 3 ME = \frac{\frac{\sqrt{3}}{4}}{\frac{1}{4} + \frac{\sqrt{3}}{2}} = \frac{\sqrt{3}}{1 + 2\sqrt{3}} , so the slope of R C = 1 + 2 3 3 . RC = -\frac{1 + 2\sqrt{3}}{\sqrt{3}}. Instead of finding the equation of the line, we use the definition of slope: for every C O = x CO = x to the left, we go x ( 1 + 2 3 ) 3 = 3 2 \frac{x(1 + 2\sqrt{3})}{\sqrt{3}} = \frac{\sqrt{3}}{2} up. Thus, x = 3 2 1 + 2 3 = 3 4 3 + 2 = 3 ( 4 3 2 ) 44 = 6 3 3 22 . x = \frac{\frac{3}{2}}{1 + 2\sqrt{3}} = \frac{3}{4\sqrt{3} + 2} = \frac{3(4\sqrt{3} - 2)}{44} = \frac{6\sqrt{3} - 3}{22}. D C = 1 2 x = 1 2 6 3 3 22 = 14 6 3 22 DC = \frac{1}{2} - x = \frac{1}{2} - \frac{6\sqrt{3} - 3}{22} = \frac{14 - 6\sqrt{3}}{22} , and A E = 7 27 22 AE = \frac{7 - \sqrt{27}}{22} , so the answer is 56 \boxed{56} .

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