Uniquity of an Equilateral

Geometry Level 4

Two lines l 1 l_1 and l 2 l_2 intersect at an angle of θ \theta . A point P P is randomly chosen in the same plane as the two lines. If there is exactly one equilateral triangle with one vertex as P P , one vertex on l 1 l_1 , and one vertex on l 2 l_2 , then find the sum of all possible values of θ \theta .

Details and Assumptions

0 θ 9 0 0\le \theta \le 90^{\circ}


The answer is 60.

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1 solution

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To have other two vertex on the two lines~~P must be symmetrically placed w.r.t. these two lines. So, it must be on the angle bisector. The angle at P must be 60. So that the other angles of the triangle be 60, the triangle with the point of intersection of the two lines also must be 60. In fact, the two triangles, are the two halves of a rhombus. ....>So the lines are at 60 degrees. The is no other solution.

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Calvin Lin Staff - 6 years, 10 months ago

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\color\red {Thanks~~ a~~ lot.} I will take help of my grand daughter. She taught me how to post a PNG file in posting a problem or a note.

Niranjan Khanderia - 6 years, 10 months ago

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If you email me the file, I can help you upload it.

Calvin Lin Staff - 6 years, 10 months ago

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@Calvin Lin I am trying. Mean while at what e.mail can I sent the file?

Niranjan Khanderia - 6 years, 10 months ago

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@Niranjan Khanderia Calvin [at] Brilliant.org

Calvin Lin Staff - 6 years, 10 months ago

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@Calvin Lin The file I have email to you is for "Hitting a Bird with Projectile"

Niranjan Khanderia - 6 years, 10 months ago

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