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Geometry Difficulty 3.8 AMC 10/12 Find the answer

Triangle ABCABC has AB=13,BC=14AB = 13, BC = 14, and AC=15AC = 15. The points D,ED, E, and FF are the midpoints of AB,BC\overline{AB}, \overline{BC}, and AC\overline{AC} respectively. Let XEX \neq E be the intersection of the circumcircles of BDE\triangle BDE and CEF\triangle CEF. What is XA+XB+XCXA + XB + XC?

Pick one

Solution

Let us also consider the circumcircle of ADF\triangle ADF.
Note that if we draw the perpendicular bisector of each side, we will have the circumcenter of ABC\triangle ABC which is PP, Also, since mADP=mAFP=90m\angle ADP = m\angle AFP = 90^\circ. ADPFADPF is cyclic, similarly, BDPEBDPE and CEPFCEPF are also cyclic. With this, we know that the circumcircles of ADF\triangle ADF, BDE\triangle BDE and CEF\triangle CEF all intersect at PP, so PP is XX.
The question now becomes calculating the sum of the distance from each vertex to the circumcenter.
We can calculate the distances with coordinate geometry. (Note that XA=XB=XCXA = XB = XC because XX is the circumcenter.)
Let A=(5,12)A = (5,12), B=(0,0)B = (0,0), C=(14,0)C = (14, 0), X=(x0,y0)X= (x_0, y_0)
Then XX is on the line x=7x = 7 and also the line with slope 512-\frac{5}{12} that passes through (2.5,6)(2.5, 6).
y0=64524=338y_0 = 6-\frac{45}{24} = \frac{33}{8}
So X=(7,338)X = (7, \frac{33}{8})
and XA+XB+XC=3XB=372+(338)2=3×658=1958XA +XB+XC = 3XB = 3\sqrt{7^2 + \left(\frac{33}{8}\right)^2} = 3\times\frac{65}{8}=\frac{195}{8}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.