[Math] Probability that the triangle is acute

geometric-probabilitygeometryprobability

A triangle is formed by randomly choosing three distinct points on the circumference of a circle and joining them.

What is the probability that the formed triangle is an acute triangle?

Best Answer

Hint: Consider the unit circle, centered at $(0,0)$ and fix the first point as $A=(-1,0)$. Consider the point $B$ in quadrant II. There must be a region of the circle where we can put our third point, $C$, such that $\triangle ABC$ is acute.

Label the points on the circle opposite $A$ and $B$ as $A'$ and $B'$, respectively. Thales' Theorem and the Inscribed Angle Theorem tell us how the angles of the triangle change when $C$ is moved around the circle. This shows us that the region (shown in green) where $C$ makes $\triangle ABC$ acute is the arc between $A'$ and $B'$. The region where $C$ forms an obtuse triangle is shown in red.

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This holds true if $B$ is in quadrant I.

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Now, constrain $B$ to quadrant II again but label its reflection in the $y$-axis as $D$. Label the point opposite $D$ on the circle as $D'$ and draw the region where $ADC$ forms an acute triangle in dashed blue.

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Hint (a): What is the average size of the arcs $A'B'$ and $A'D'$ as a proportion of the circle?

Hint (b): What does this tell us about the average size of the region where the triangle is acute, if we randomly select a point on the upper semicircle?

Hint (c): Does this generalise to when $B$ is in the bottom semicircle? And to when $A$ is not $(-1,0)$?

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