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Geometry Difficulty 7.3 National olympiad, round 2 Find the answer

We draw two lines (1),(2)(\ell_1) , (\ell_2) through the orthocenter HH of the triangle ABCABC such that each one is dividing the triangle into two figures of equal area and equal perimeters. Find the angles of the triangle.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We are given that the lines (1)(\ell_1) and (2)(\ell_2) pass through the orthocenter HH of triangle ABCABC and each line divides the triangle into two figures of equal area and equal perimeters. We need to determine the angles of the triangle.

The orthocenter HH of a triangle is the intersection of its altitudes. For the line (1)(\ell_1) to divide the triangle ABCABC into two parts of equal area, it must pass through HH and reach the midpoints of the sides of the triangle. Similarly, line (2)(\ell_2) must satisfy the same condition. If both lines divide the triangle into regions of equal perimeter as well as equal area, this implies symmetry.

For the configuration where such conditions hold, consider an equilateral triangle:

1. In an equilateral triangle with sides aa, all altitudes are equal, and the medians and altitudes coincide. The orthocenter HH is the same as the centroid and the circumcenter.

2. If line (1)(\ell_1) passes through HH, it can align with any median (which is also an altitude). Given the symmetry, the division will always result in parts with equal area and perimeter.

3. Similarly, line (2)(\ell_2) can align with another median. In an equilateral triangle, any line through the orthocenter divides the triangle into regions of equal area and perimeter due to its symmetry.

When equilateral conditions are not satisfied, such divisions generally do not hold, as the perimeters of resulting sections would differ once they form different shaped sections other than those symmetric to each other, distinct in non-equilateral triangles.

Therefore, the only triangle for which two such lines exist is an equilateral triangle. Thus, every angle in the triangle must be:
60 \boxed{60^\circ}

Hence, the angles of the triangle are 60,60,6060^\circ, 60^\circ, 60^\circ.

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