Gergonne's Go

Geometry Level 5

The points A , B A,B lie on the diameter of a unit circle, and C C is a third point on that circle, forming a triangle A B C ABC . The locus of the Gergonne point of the triangle A B C ABC forms a closed lens-shaped curve (marked in purple on the diagram). It can be shown that the area enclosed by the locus is equal to α π + p β γ q + p q δ q cos 1 ( ε ζ p ) -\alpha \pi + \frac{p^\beta \gamma}{q} + \frac{p^q \delta}{\sqrt{q}} \cos^{-1}\left(\frac{\varepsilon}{\zeta}\sqrt{p}\right) where α , β , γ , δ , ε , ζ \alpha,\beta,\gamma,\delta,\varepsilon,\zeta are positive integers (with ε \varepsilon and ζ \zeta coprime) and p , q p,q are distinct prime numbers.

Find the value of α + β + γ + δ + ε + ζ + p + q \alpha + \beta + \gamma + \delta + \varepsilon + \zeta + p + q .


The answer is 117.

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1 solution

Mark Hennings
May 6, 2021

If we let B A C = θ \angle BAC = \theta for 0 < θ < 1 2 π 0 < \theta < \tfrac12\pi (so that C C is in the upper semicircle), then the vertices of the triangle A B C ABC are A ( 1 , 0 ) B ( 1 , 0 ) C ( cos 2 θ , sin 2 θ ) A\; (-1,0) \hspace{1cm} B\;(1,0) \hspace{1cm} C\; (\cos2\theta,\sin2\theta) The triangle A B C ABC has sides a , b , c a,b,c and semiperimeter s s where a = 2 sin θ b = 2 cos θ c = 2 s = 1 + cos θ + sin θ a \; = \; 2\sin\theta \hspace{1cm} b \; = \; 2\cos\theta \hspace{1cm} c \; = \; 2 \hspace{1cm} s \; = \; 1 + \cos\theta + \sin\theta If D , E , F D,E,F are the touch points of the incircle with the sides of the triangle, then A F = A E = s a AF=AE=s-a and B D = s b BD=s-b , and hence we have coordinates D ( 1 ( 1 + sin θ cos θ ) sin θ , ( 1 + sin θ cos θ ) cos θ ) E ( 1 + ( 1 + cos θ sin θ ) cos θ , ( 1 + cos θ sin θ ) sin θ ) F ( cos θ sin θ , 0 ) \begin{array}{lll} D & \hspace{0.2cm} & \big(1 - (1 + \sin\theta - \cos\theta)\sin\theta,(1 + \sin\theta - \cos\theta)\cos\theta\big) \\ E & & \big(-1 + (1 + \cos\theta - \sin\theta)\cos\theta,(1 + \cos\theta - \sin\theta)\sin\theta\big) \\ F & & (\cos\theta - \sin\theta,0) \end{array} and the inradius of the triangle A B C ABC is r = s c = cos θ + sin θ 1 r = s-c = \cos\theta + \sin\theta - 1 . Solving simultaneously the equations for the lines A D AD and B E BE , we find that the Gergonne point G e Ge has coordinates ( X ( θ ) , Y ( θ ) ) \big(X(\theta),Y(\theta)\big) , where X ( θ ) = cos 2 θ 2 ( cos θ sin θ ) ( cos θ + sin θ 1 ) 3 + cos θ + sin θ Y ( θ ) = ( cos θ + sin θ 1 ) ( 1 + cos θ + sin θ ) 2 3 + cos θ + sin θ \begin{aligned} X(\theta) & = \; \cos2\theta - \frac{2(\cos\theta - \sin\theta)(\cos\theta + \sin\theta - 1)}{3 + \cos\theta + \sin\theta} \\ Y(\theta) & = \; \frac{(\cos\theta + \sin\theta - 1) (1 + \cos\theta + \sin\theta)^2}{3 + \cos\theta + \sin\theta} \end{aligned} By symmetry, the desired area is Δ = 2 1 2 π 0 X ( θ ) Y ( θ ) d θ = 2 1 2 π 0 ( 1 + c o s θ + sin θ ) ( 1 + cos θ + sin θ ) 3 ( 6 + cos θ + cos 3 θ + sin θ 8 sin 2 θ sin 3 θ ) ( 3 + cos θ + sin θ ) 3 d θ = 32 0 1 ( t 1 ) t ( 1 + t ) 3 ( 1 + 9 t + 8 t 2 6 t 3 + t 4 7 t 5 + 2 t 6 ) ( 1 + t 2 ) 5 ( 2 + t + t 2 ) 3 d t \begin{aligned} \Delta & = \; 2\int_{\frac12\pi}^0 X'(\theta)Y(\theta)\,d\theta \\ & = \; 2\int_{\frac12\pi}^0 \frac{(-1 + cos\theta + \sin\theta) (1 + \cos\theta + \sin\theta)^3 (-6 +\cos\theta + \cos3\theta + \sin\theta - 8\sin2\theta - \sin3\theta)}{(3 + \cos\theta + \sin\theta)^3} \,d\theta \\ & = \; -32\int_0^1 \frac{(t - 1) t (1 + t)^3 (1 + 9 t + 8 t^2 - 6 t^3 + t^4 - 7 t^5 + 2 t^6)}{(1 + t^2)^5 (2 + t + t^2)^3} \,dt \end{aligned} using the t = tan 1 2 θ t = \tan\tfrac12\theta substitution. This last integrand can be expanded by partial fractions as 32 ( 16 ( 1 + t 2 ) 5 + 4 ( 3 t 11 ) ( 1 + t 2 ) 4 2 ( 14 t 19 ) ( 1 + t 2 ) 3 + 16 t 5 ( 1 + t 2 ) 2 + 3 ( 1 + t 2 ) 14 ( t 1 ) ( 2 + t + t 2 ) 3 19 t + 33 ( 2 + t + t 2 ) 2 3 ( 2 + t + t 2 ) ) -32\left(\begin{array}{l} \displaystyle \frac{16}{(1 + t^2)^5} + \frac{4 (3t - 11)}{(1 + t^2)^4} - \frac{ 2 (14 t - 19)}{(1 + t^2)^3} + \frac{16 t - 5}{(1 + t^2)^2} + \frac{3}{( 1 + t^2)} \\[2ex] \displaystyle - \frac{14 (t - 1)}{(2 + t + t^2)^3} - \frac{19 t + 33}{(2 + t + t^2)^2} - \frac{3}{( 2 + t + t^2)} \end{array}\right) Since each of these terms can be integrated using either t = tan φ t = \tan\varphi or t + 1 2 = tan φ t + \tfrac12 = \tan\varphi as substitution, we deduce that Δ = 43 π + 368 7 + 3200 7 cos 1 ( 5 8 2 ) = 43 π + 2 4 × 23 7 + 2 7 × 25 7 cos 1 ( 5 8 2 ) \begin{aligned} \Delta & = \; -43\pi + \frac{368}{7} +\frac{3200}{\sqrt{7}}\cos^{-1}\big(\tfrac58\sqrt{2}\big) \\ & = \; -43\pi + \frac{2^4 \times 23}{7} + \frac{2^7 \times 25}{\sqrt{7}}\cos^{-1}\big(\tfrac58\sqrt{2}\big) \end{aligned} so that α = 43 \alpha = 43 , β = 4 \beta = 4 , γ = 23 \gamma = 23 , δ = 25 \delta = 25 , ε = 5 \varepsilon = 5 , ζ = 8 \zeta = 8 , p = 2 p = 2 and q = 7 q=7 , so that α + β + γ + δ + ε + ζ + p + q = 117 \alpha +\beta + \gamma + \delta + \varepsilon + \zeta + p + q = \boxed{117} .

@Mark Hennings So that means that it is equal to the phrases, 'so as to', and 'in order to'.

. . - 1 month ago

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