Olympiad Maths Prep

Track / Stage 6 / 333 of 400 #1333 of 2000

Problem 1333

National olympiad, first round
Geometry Difficulty 6.6 Find the answer

What is the area of the region in the complex plane consisting of all points zz satisfying both 1z1<1|\tfrac{1}{z}-1|<1 and z1<1|z-1|<1? (z|z| denotes the magnitude of a complex number, i.e. a+bi=a2+b2|a+bi|=\sqrt{a^2+b^2}.)

Official solution

1. Interpret the inequalities geometrically:
- The inequality z1<1 |z - 1| < 1 describes the interior of a unit circle centered at 1 1 in the complex plane.
- The inequality 1z1<1 \left| \frac{1}{z} - 1 \right| < 1 can be rewritten by clearing the denominator. Let z=x+yi z = x + yi where x x and y y are real numbers.

2. Rewrite the first inequality:
1z1<1    1zz<1    1z<z \left| \frac{1}{z} - 1 \right| < 1 \implies \left| \frac{1 - z}{z} \right| < 1 \implies |1 - z| < |z|
This inequality 1z<z |1 - z| < |z| is satisfied by all z z with real part (z)>12 \Re(z) > \frac{1}{2} . This can be shown by considering the geometric interpretation of the magnitudes.

3. Combine the regions:
- The region z1<1 |z - 1| < 1 is a unit circle centered at 1 1 .
- The region 1z<z |1 - z| < |z| is the half-plane where the real part of z z is greater than 12 \frac{1}{2} .

4. Find the intersection:
- The intersection of these two regions is the part of the unit circle centered at 1 1 that lies to the right of the vertical line (z)=12 \Re(z) = \frac{1}{2} .

5. Calculate the area:
- The intersection forms a sector of the circle and a triangle.
- The angle of the sector is 2π3 \frac{2\pi}{3} radians (since the line (z)=12 \Re(z) = \frac{1}{2} intersects the circle at angles ±π3 \pm \frac{\pi}{3} from the positive real axis).
- The area of the sector is:
Area of sector=12×(radius)2×(angle)=12×12×2π3=π3 \text{Area of sector} = \frac{1}{2} \times (\text{radius})^2 \times (\text{angle}) = \frac{1}{2} \times 1^2 \times \frac{2\pi}{3} = \frac{\pi}{3}
- The area of the triangle formed by the intersection points and the center of the circle is:
Area of triangle=12×(base)×(height)=12×1×32=34 \text{Area of triangle} = \frac{1}{2} \times (\text{base}) \times (\text{height}) = \frac{1}{2} \times 1 \times \frac{\sqrt{3}}{2} = \frac{\sqrt{3}}{4}

6. Sum the areas:
Total area=π3+34 \text{Total area} = \frac{\pi}{3} + \frac{\sqrt{3}}{4}

The final answer is π3+34\boxed{\frac{\pi}{3} + \frac{\sqrt{3}}{4}}

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