lie on the diameter of a unit circle, and is a third point on that circle, forming a triangle . The locus of the Gergonne point of the triangle 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 where are positive integers (with and coprime) and are distinct prime numbers.
The pointsFind the value of .
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If we let ∠ B A C = θ for 0 < θ < 2 1 π (so that C is in the upper semicircle), then the vertices of the triangle A B C are A ( − 1 , 0 ) B ( 1 , 0 ) C ( cos 2 θ , sin 2 θ ) The triangle A B C has sides a , b , c and semiperimeter s where a = 2 sin θ b = 2 cos θ c = 2 s = 1 + cos θ + sin θ If D , E , F are the touch points of the incircle with the sides of the triangle, then A F = A E = s − a and B D = s − b , and hence we have coordinates D E F ( 1 − ( 1 + sin θ − cos θ ) sin θ , ( 1 + sin θ − cos θ ) cos θ ) ( − 1 + ( 1 + cos θ − sin θ ) cos θ , ( 1 + cos θ − sin θ ) sin θ ) ( cos θ − sin θ , 0 ) and the inradius of the triangle A B C is r = s − c = cos θ + sin θ − 1 . Solving simultaneously the equations for the lines A D and B E , we find that the Gergonne point G e has coordinates ( X ( θ ) , Y ( θ ) ) , where X ( θ ) Y ( θ ) = cos 2 θ − 3 + cos θ + sin θ 2 ( cos θ − sin θ ) ( cos θ + sin θ − 1 ) = 3 + cos θ + sin θ ( cos θ + sin θ − 1 ) ( 1 + cos θ + sin θ ) 2 By symmetry, the desired area is Δ = 2 ∫ 2 1 π 0 X ′ ( θ ) Y ( θ ) d θ = 2 ∫ 2 1 π 0 ( 3 + cos θ + sin θ ) 3 ( − 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 ) d θ = − 3 2 ∫ 0 1 ( 1 + t 2 ) 5 ( 2 + t + t 2 ) 3 ( t − 1 ) t ( 1 + t ) 3 ( 1 + 9 t + 8 t 2 − 6 t 3 + t 4 − 7 t 5 + 2 t 6 ) d t using the t = tan 2 1 θ substitution. This last integrand can be expanded by partial fractions as − 3 2 ⎝ ⎜ ⎜ ⎛ ( 1 + t 2 ) 5 1 6 + ( 1 + t 2 ) 4 4 ( 3 t − 1 1 ) − ( 1 + t 2 ) 3 2 ( 1 4 t − 1 9 ) + ( 1 + t 2 ) 2 1 6 t − 5 + ( 1 + t 2 ) 3 − ( 2 + t + t 2 ) 3 1 4 ( t − 1 ) − ( 2 + t + t 2 ) 2 1 9 t + 3 3 − ( 2 + t + t 2 ) 3 ⎠ ⎟ ⎟ ⎞ Since each of these terms can be integrated using either t = tan φ or t + 2 1 = tan φ as substitution, we deduce that Δ = − 4 3 π + 7 3 6 8 + 7 3 2 0 0 cos − 1 ( 8 5 2 ) = − 4 3 π + 7 2 4 × 2 3 + 7 2 7 × 2 5 cos − 1 ( 8 5 2 ) so that α = 4 3 , β = 4 , γ = 2 3 , δ = 2 5 , ε = 5 , ζ = 8 , p = 2 and q = 7 , so that α + β + γ + δ + ε + ζ + p + q = 1 1 7 .