Maths Olympiad Prep

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Geometry Difficulty 5.6 AIME, harder Prove it Philippines

Problem:
Find the probability of obtaining two numbers xx and yy in the interval [0,1][0,1] such that x23xy+2y2>0x^{2}-3 x y+2 y^{2}>0.

Solution

Solution:
Let x,y[0,1]x, y \in [0,1]. We are to find the probability that x23xy+2y2>0x^{2} - 3 x y + 2 y^{2} > 0.

We can factor the quadratic:
x23xy+2y2=(xy)(x2y) x^{2} - 3 x y + 2 y^{2} = (x - y)(x - 2y)
So, (xy)(x2y)>0(x - y)(x - 2y) > 0.

This inequality holds if both factors are positive or both are negative:

Case 1: xy>0x - y > 0 and x2y>0x - 2y > 0

This gives x>yx > y and x>2yx > 2y.
But x>2yx > 2y implies x>yx > y, so the only restriction is x>2yx > 2y.
But x1x \leq 1, y0y \geq 0, and x>2yx > 2y.

For yy in [0,0.5)[0, 0.5), xx runs from 2y2y to 11.

Case 2: xy<0x - y < 0 and x2y<0x - 2y < 0

This gives x<yx < y and x<2yx < 2y.
But x<yx < y implies x<2yx < 2y (since y<2yy < 2y for y>0y > 0), so the only restriction is x<yx < y.

For yy in [0,1][0,1], xx runs from 00 to yy.

So, the region is:
- For yy in [0,0.5][0, 0.5], xx in [2y,1][2y, 1] (from Case 1)
- For yy in [0,1][0,1], xx in [0,y][0, y] (from Case 2)

The total area (probability) is:

A=A1+A2A = A_1 + A_2

Where
A1=y=00.5(12y)dy A_1 = \int_{y=0}^{0.5} (1 - 2y) \, dy
A2=y=01ydy A_2 = \int_{y=0}^{1} y \, dy

Compute A1A_1:
A1=00.5(12y)dy=[yy2]00.5=(0.50.25)(00)=0.25 A_1 = \int_{0}^{0.5} (1 - 2y) \, dy = \left[ y - y^2 \right]_{0}^{0.5} = (0.5 - 0.25) - (0 - 0) = 0.25

Compute A2A_2:
A2=01ydy=[y22]01=12 A_2 = \int_{0}^{1} y \, dy = \left[ \frac{y^2}{2} \right]_{0}^{1} = \frac{1}{2}

Therefore, the total probability is:
A=0.25+0.5=0.75 A = 0.25 + 0.5 = 0.75

So, the answer is 34\boxed{\dfrac{3}{4}}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.