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Geometry Difficulty 5.8 AIME, harder Prove it

Example 1 As shown in the figure, O\odot O intersects the sides BC,CA,ABBC, CA, AB of ABC\triangle ABC at points A1A_{1} and A2A_{2}, points B1B_{1} and B2B_{2}, points C1C_{1} and C2C_{2}, respectively. Given that the perpendiculars from points A1A_{1}, B1B_{1}, C1C_{1} to BCBC, CACA, ABAB intersect at point PP. Prove: The perpendiculars from points A2A_{2}, B2B_{2}, C2C_{2} to BCBC, CACA, ABAB also intersect at one point.

Solution

Proof: Taking point OO as the origin and C2B2C_{2} B_{2} as the xx-axis to establish a Cartesian coordinate system. Let the line passing through point A2A_{2} and perpendicular to BCB C be denoted as j'j. The line passing through point B2B_{2} and perpendicular to ACA C intersects at point QQ. Clearly, the two lines passing through points A1A_{1} and A2A_{2} and perpendicular to BCB C are symmetric about the yy-axis; similarly, the two lines passing through points B1B_{1} and B2B_{2} and perpendicular to ACA C are symmetric about the line passing through point OO and perpendicular to ACA C; the same applies to points C1C_{1} and C2C_{2}.

Let the equation of line lPA1l_{P A_{1}} be x=mx = -m,
and the equation of line lPB1l_{P B_{1}} be y=kx+by = kx + b.
Then the equation of line lOA2l_{O A_{2}} is x=mx = m,
and the equation of line lOB2l_{O B_{2}} is y=kxby = kx - b.
Therefore, the family of lines passing through point PP is given by
λ(kxy+b)+μ(x+m)=0, \lambda(k x - y + b) + \mu(x + m) = 0,

and PC1P C_{1} is one of these lines.
Let the equation of line PC1P C_{1} be
(λ0k+μ0)xλ0y+λ0b+μ0m=0. \left(\lambda_{0} k + \mu_{0}\right) x - \lambda_{0} y + \lambda_{0} b + \mu_{0} m = 0.

Thus, the equation of the line passing through point C2C_{2} and perpendicular to ABA B is
(λ0k+μ0)xλ0yλ0bμ0m=0. \left(\lambda_{0} k + \mu_{0}\right) x - \lambda_{0} y - \lambda_{0} b - \mu_{0} m = 0.

The family of lines passing through point QQ is given by
λ(kxyb)+μ(xm)=0. \lambda^{\prime}(k x - y - b) + \mu^{\prime}(x - m) = 0.

When λ=λ0\lambda^{\prime} = \lambda_{0} and μ=μ0\mu^{\prime} = \mu_{0}, the above equation is obtained, meaning that the line passing through point C2C_{2} and perpendicular to ABA B also passes through point QQ.

Therefore, the perpendiculars drawn from points A2A_{2}, B2B_{2}, and C2C_{2} to BCB C, CAC A, and ABA B respectively intersect at a single point QQ.

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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.