Maths Olympiad Prep

Track / Stage 6 / 184 of 400 #1184 of 1964

Problem 1184

National olympiad, first round
Combinatorics Difficulty 6.3 Find the answer

Bread draws a circle. He then selects four random distinct points on the circumference of the circle to form a convex quadrilateral. Kwu comes by and randomly chooses another 3 distinct points (none of which are the same as Bread's four points) on the circle to form a triangle. Find the probability that Kwu's triangle does not intersect Bread's quadrilateral, where two polygons intersect if they have at least one pair of sides intersecting.

Proposed by Nathan Cho

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Official solution

1. Plotting the Points:
- We start by plotting all 7 points on the circumference of the circle. Let's label these points as A1,A2,A3,A4,A5,A6,A7 A_1, A_2, A_3, A_4, A_5, A_6, A_7 .

2. Labeling the Points:
- Out of these 7 points, Bread selects 4 points to form a quadrilateral. Let's label these points as Q1,Q2,Q3,Q4 Q_1, Q_2, Q_3, Q_4 .
- Kwu then selects the remaining 3 points to form a triangle. Let's label these points as P1,P2,P3 P_1, P_2, P_3 .

3. Condition for Non-Intersection:
- For Kwu's triangle to not intersect Bread's quadrilateral, the 3 points P1,P2,P3 P_1, P_2, P_3 must be consecutive on the circle. This ensures that the triangle formed by P1,P2,P3 P_1, P_2, P_3 lies entirely within one of the arcs formed by the quadrilateral Q1,Q2,Q3,Q4 Q_1, Q_2, Q_3, Q_4 .

4. Counting Consecutive Points:
- There are 7 points on the circle. We need to count the number of ways to choose 3 consecutive points out of these 7 points.
- The number of ways to choose 3 consecutive points from 7 points is 7. This is because we can start at any of the 7 points and choose the next two points in a clockwise direction.

5. Total Combinations:
- The total number of ways to choose any 3 points out of 7 is given by the binomial coefficient (73) \binom{7}{3} .
- (73)=7!3!(73)!=7×6×53×2×1=35 \binom{7}{3} = \frac{7!}{3!(7-3)!} = \frac{7 \times 6 \times 5}{3 \times 2 \times 1} = 35

6. Calculating the Probability:
- The probability that Kwu's triangle does not intersect Bread's quadrilateral is the ratio of the number of favorable outcomes (choosing 3 consecutive points) to the total number of outcomes (choosing any 3 points out of 7).
- Probability=Number of ways to choose 3 consecutive pointsTotal number of ways to choose 3 points=735=15 \text{Probability} = \frac{\text{Number of ways to choose 3 consecutive points}}{\text{Total number of ways to choose 3 points}} = \frac{7}{35} = \frac{1}{5}

The final answer is 15\boxed{\frac{1}{5}}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.