Nagel Angle Ankle

Geometry Level 5

The animation on the right shows the circle inscribing the triangle whose one of the sides is the diameter with two fixed endpoints. The point is selected and rotated counterclockwise on the circumference, so that the Nagel point of the triangle forms the locus colored in purple.

If the following area ratio is

R = area bounded inside the locus area of the whole circle R = \dfrac{\text{area bounded inside the locus}}{\text{area of the whole circle}}

Input 1 0 6 R \lfloor 10^6 R \rfloor as your answer.


The answer is 92958.

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1 solution

Mark Hennings
May 4, 2021

If the points A , B , C A,B,C have coordinates ( 1 , 0 ) (-1,0) , ( 1 , 0 ) (1,0) and ( cos 2 θ , sin 2 θ ) (\cos2\theta,\sin2\theta) respectively (we are writing B A C = θ \angle BAC = \theta ), then the Nagel points N a Na is the intersection of the lines A D AD and B E BE , where 2 + A E = 2 + B D 2 + AE = 2 + BD is equal to the semiperimeter 1 + cos θ + sin θ 1+\cos\theta+\sin\theta of the triangle A B C ABC . This means that D ( 1 ( sin θ + cos θ 1 ) sin θ , ( sin θ + cos θ 1 ) cos θ ) E ( 1 + ( sin θ + cos θ 1 ) cos θ , ( sin θ + cos θ 1 ) sin θ ) \begin{aligned} D\;& \big(1 - (\sin\theta+\cos\theta-1)\sin\theta,(\sin\theta+\cos\theta-1)\cos\theta\big) \\ E\;&\big(-1 + (\sin\theta+\cos\theta-1)\cos\theta,(\sin\theta+\cos\theta-1)\sin\theta\big) \end{aligned} and hence the Nagel point has coordinates N a ( x ( θ ) , y ( θ ) ) = ( cos 2 θ + 2 sin θ 2 cos θ , ( sin θ + cos θ 1 ) 2 ) Na\; \big(x(\theta),y(\theta)\big) \; = \; \big(\cos2\theta + 2\sin\theta - 2\cos\theta,(\sin\theta + \cos\theta - 1)^2\big) and so the upper half of the area enclosed by the locus of the Nagel point is A = 0 1 2 π x ( θ ) y ( θ ) d θ = 8 5 2 π A \; = \; \int_0^{\frac12\pi} x'(\theta)y(\theta)\,d\theta \; = \; 8 - \tfrac52\pi Thus the desired proportion is R = 2 A π = 16 π 5 = 0.09295817894... R \; = \; \tfrac{2A}{\pi} \; = \; \tfrac{16}{\pi} - 5 \; = \; 0.09295817894... and hence 1 0 6 R = 92958 \lfloor 10^6 R \rfloor \,=\, \boxed{92958} .

@Mark Hennings , we really liked your comment, and have converted it into a solution.

Brilliant Mathematics Staff - 1 month, 1 week ago

@Mark Hennings , first of all, as always, great solution, sir! My approach was exactly the same as yours (I wonder if someone tried to determine y ( x ) y(x) explicitly).

I have a question to you: How did you find the coordinates of the Nagel point, having found the coordinates of D D and E E ? I went all the way through of finding the equations of the lines A D AD and B E BE and then solving the resulting system (as N a = A D B E Na=AD\cap BE ). The trigonometric expression were a huge mess so I mistook somewhere in the process and got a wrong result.

So...did you do it any smarter than me or you were much more accurate transforming these nasty expressions?

Veselin Dimov - 1 month ago

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