AMC 10 · 2015 · #25

Grade 8 geometry-2d

Archetype Geometric Probability by Area └ Sub-archetype Uniform Draws to Favorable Region

geometric-probabilityprobability-basicarea-circles casework ↑ Prerequisites: geometric-probability

📏 Long solution 💡 4 insights

Problem

A square has side length 1. Two points are dropped independently and uniformly at random somewhere on the square's boundary (its four sides). Find the chance that the straight-line distance between the two points is at least 1/2. The answer comes out as a-bπ/c in lowest terms, and you report a+b+c.

Pick an answer.

(A)

(B)

(C)

(D)

(E)

AMC 10 2015 problem © Mathematical Association of America (MAA AMC). Reproduced for educational use.

How to solve

Strategy Identify Subproblems

The two points can land on the same side, on two sides that share a corner, or on two sides that face each other, and the distance behaves completely differently in each situation. Tool #7 (Identify Subproblems) splits the problem along exactly those three cases and lets symmetry fix the chance of each case. Inside a case, Tool #4 (Introduce a Variable) names each point's position by how far it sits from a corner, turning "distance at least 1/2" into an inequality in two coordinates. Tool #1 (Draw a Diagram) plots those two coordinates on a unit square so a probability becomes an area. In the corner-sharing case the qualifying region is awkward, so Tool #16 (Count the Complement) measures the small quarter-circle that fails instead and subtracts it. Weighting the three case-probabilities by how often each case happens assembles the final answer.

1STEP 1

Split by where the points land

Fix the first point; the second lands on the same, an adjacent (2/4=1/2), or the opposite side — handle each separately.

P(same)=1/4, P(adjacent)=1/2, P(opposite)=1/4

2STEP 2

Same side: an area model

Same-side positions form a random point in the unit square, so the qualifying region's area gives probability 1/4.

P_same=P (|x-y| ≥ 1/2)=2·1/2·1/2·1/2=1/4

3STEP 3

Adjacent sides: set up the distance

Set the shared corner as the origin so each point's distance becomes a Pythagorean hypotenuse in x and y.

distance²=x²+y², distance ≥ 1/2 ⇔ x²+y² ≥ 1/4

4STEP 4

Adjacent sides: subtract the failing quarter-disk

The failing region is a quarter-disk of radius 1/2, so subtracting its area leaves probability 1-π/16.

P_adj=1-(1/4·π(1/2)²)/1=1-π/16

5STEP 5

Opposite sides: always far enough

Opposite-side points are always at least 1 apart, well past 1/2, so this case's probability is 1.

distance ≥ 1 ≥ 1/2 → P_opp=1

6STEP 6

Average the cases and read off the answer

Weighting the three cases 1/4, 1/2, 1/4 and summing gives a+b+c=59, answer (A).

P=1/4·1/4+1/2 (1-π/16)+1/4·1=(26-π)/32 → a+b+c=59 (A)

Answer

The result 26-π/32≈26-3.14/32≈22.86/32≈ 0.71 is a sensible probability strictly between 0 and 1. It also sits correctly between the three case values it averages: 0.25 (same side), 1-π/16≈0.80 (adjacent), and 1 (opposite), and it leans toward the larger ones because the adjacent and opposite cases together carry 3/4 of the weight. Finally gcd(26,1,32)=1, so the form a-bπ/c is genuinely reduced and a+b+c=59 is forced, matching (A).

💡Key takeaway

Sort the two points by which sides they land on, turn each case into an area on a unit square, then average by how often it happens.

Split by where the points land
Same side: an area model
Adjacent sides: set up the distance
Adjacent sides: subtract the failing quarter-disk
Opposite sides: always far enough
Average the cases and read off the answer