All Triangles Are Isosceles

Geometry Level 5

Let Δ A B C \Delta ABC be an arbitrary triangle.

Point P P is the midpoint of line B C BC , and line P Q PQ is perpendicular to line B C BC .

Line A Q AQ bisects angle B A C \angle BAC .

Lines A B AB and A C AC are extended as necessary to include points R R and S S such that Δ A Q R \Delta AQR and Δ A Q S \Delta AQS are right triangles.

By reason of symmetry, line segments B Q BQ and C Q CQ are equal.

By reason of symmetry, line segments R Q RQ and S Q SQ are equal.

Therefore, by reason of congruent right triangles, line segments B R BR and C S CS are equal.

But, also by reason of congruent right triangles, line segments A R AR and A S AS are also equal.

Therefore, line segments A B AB and A C AC are equal, and triangle Δ A B C \Delta ABC is isosceles.

Which of the following multiple choice answers is correct?


Note : This problem is a refugee from Brilliant April Fools' Day
All triangles are isosceles triangles Lines P Q PQ and A Q AQ do not intersect outside triangle Δ A B C \Delta ABC Right triangles Δ A Q R \Delta AQR and Δ A Q S \Delta AQS cannot always be constructed "By reason of symmetry" is not a valid argument in synthetic geometry Right triangles Δ Q R B \Delta QRB and Δ Q S C \Delta QSC are not congruent None of the other answers Right triangles Δ A Q R \Delta AQR and Δ A Q S \Delta AQS are not congruent Lines P Q PQ and A Q AQ do not intersect anywhere

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1 solution

Michael Mendrin
Apr 1, 2016

If line segment A C AC is longer than A B AB , then point S S will fall between A A and C C . The graphic shows point S S as being outside of line segment A C AC , which is impossible. Both right triangles Δ A Q R \Delta AQR and Δ A Q S \Delta AQS can be constructed, but just not with both points R R and S S outside of line segments A B AB and A C AC . See below for an accurately drawn graphic of the same.

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