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Geometry Difficulty 8.8 Shortlist Prove it Balkan Mathematical Olympiad

Let A0B0C0A_0B_0C_0 be a triangle with area equal to 2\sqrt{2}. We consider the excenters A1A_1, B1B_1 and C1C_1 then we consider the excenters, say A2A_2, B2B_2 and C2C_2, of the triangle A1B1C1A_1B_1C_1. By continuing this procedure, examine if it is possible to arrive to a triangle AnBnCnA_nB_nC_n with all coordinates rational.

Solution

The answer is no. Suppose that it is possible. We assert that the previous triangle An1Bn1Cn1A_{n-1}B_{n-1}C_{n-1} has rational coordinates. In fact, the points An1A_{n-1}, Bn1B_{n-1}, Cn1C_{n-1} are the feet of the altitudes of the triangle AnBnCnA_nB_nC_n. Therefore it is enough to show that, if a line segment has its ends with rational coordinates, then the foot of the perpendicular line passing through a point of the plane with rational coordinates has also rational coordinates. This really happens because the coordinates (x,y)(x, y) of the foot of the perpendicular are the solutions of the system y=ax+by = ax + b, y=1ax+cy = -\frac{1}{a}x + c with a,b,ca, b, c rational. Therefore, every time in the previous step the coordinates must be rational and so, we arrive to the conclusion that the coordinates of the triangle A0B0C0A_0B_0C_0 must be rational. Then from the area formula using coordinates of the vertices we find that the area of the triangle is a rational number. This contradicts the supposition that the area of the triangle is equal to 2\sqrt{2}.

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