Maths Olympiad Prep

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Geometry Difficulty 6.7 National olympiad Find the answer

Suppose ABCDABCD is a rectangle whose diagonals meet at EE. The perimeter of triangle ABEABE is 10π10\pi and the perimeter of triangle ADEADE is nn. Compute the number of possible integer values of nn.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Understanding the Problem:
- We are given a rectangle ABCDABCD with diagonals intersecting at EE.
- The perimeter of triangle ABEABE is 10π10\pi.
- We need to find the number of possible integer values for the perimeter of triangle ADEADE, denoted as nn.

2. Setting Up the Equations:
- Let aa and bb be the lengths of the sides ABAB and ADAD of the rectangle, respectively.
- The diagonals of the rectangle are equal, and they intersect at EE, the midpoint of each diagonal.
- The length of each diagonal is a2+b2\sqrt{a^2 + b^2}.

3. **Perimeter of Triangle ABEABE:**
- The perimeter of ABE\triangle ABE is given by:
a+a2+b2+a2+b22=10π a + \sqrt{a^2 + b^2} + \frac{\sqrt{a^2 + b^2}}{2} = 10\pi
- Simplifying, we get:
a+3a2+b22=10π a + \frac{3\sqrt{a^2 + b^2}}{2} = 10\pi

4. **Perimeter of Triangle ADEADE:**
- The perimeter of ADE\triangle ADE is given by:
b+a2+b2+a2+b22=n b + \sqrt{a^2 + b^2} + \frac{\sqrt{a^2 + b^2}}{2} = n
- Simplifying, we get:
b+3a2+b22=n b + \frac{3\sqrt{a^2 + b^2}}{2} = n

5. **Analyzing the Range of nn:**
- As a0a \rightarrow 0, b10πb \rightarrow 10\pi, and thus:
n20π n \rightarrow 20\pi
- As b0b \rightarrow 0, a10πa \rightarrow 10\pi, and thus:
n5π n \rightarrow 5\pi

6. Finding the Integer Values:
- The range of nn is from 5π5\pi to 20π20\pi.
- Converting to integer values:
20π5π+1 \lfloor 20\pi \rfloor - \lceil 5\pi \rceil + 1
- Approximating π3.14\pi \approx 3.14:
20×3.14=62.8=62 \lfloor 20 \times 3.14 \rfloor = \lfloor 62.8 \rfloor = 62
5×3.14=15.7=16 \lceil 5 \times 3.14 \rceil = \lceil 15.7 \rceil = 16
- Therefore, the number of integer values is:
6216+1=47 62 - 16 + 1 = 47

The final answer is 47\boxed{47}.

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