Chapter 3: Problem 46
The locus of the points representing the complex numbers which satisfy \(|z|-2=0,|z-i|-|z+5 i|=0\) is: (A) a circle with centre at origin (B) a straight line passing through origin (C) the single point \((0,-2)\) (D) None of these
Short Answer
Expert verified
(C) The single point (0, -2).
Step by step solution
01
Understanding the First Equation
The first condition \(|z|-2=0\) implies \(|z|=2\). In the complex plane, this represents a circle with center at the origin \(0,0\) and radius equal to 2.
02
Analyzing the Second Equation
The second condition \(|z-i| - |z+5i| = 0\) means the distances from a point on the complex plane to \(0,1\) and \(0,-5\) are equal. This is the definition of the perpendicular bisector of the segment joining \(0,1\) and \(0,-5\).
03
Finding the Perpendicular Bisector
The perpendicular bisector of the line segment from \(0,1\) to \(0,-5\) is a horizontal line parallel to the real axis, equidistant from both points. Calculating the midpoint between these points gives \(0,-2\), this is the line y = -2.
04
Intersection of Circle and Line
Now we consider both the circle \(|z|=2\) and the line y = -2. To find their intersection, we check which points satisfy both conditions. At \(x, y = -2\), we calculate \(|z|=\sqrt{x^2 + (-2)^2}=2\). Substituting in the circle's equation gives \(|z|=\sqrt{x^2+4}=2\). Solving \(x^2 + 4 = 4\) leads to \(x^2=0\), hence \(x=0\).
05
Solution Interpretation
The only point that lies both on the circle and on the line is \(x, y = 0, -2\). Therefore, the locus is the single point \((0, -2)\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Locus
In geometry and complex number theory, the term **locus** refers to the set of all points that satisfy a particular condition or equation. Here, the locus is utilized to find points on the complex plane that meet specific criteria. For this exercise, there are two conditions given:
- The modulus of the complex number, \(|z|-2=0\), establishing a circle centered at the origin.
- The equation \(|z-i| - |z+5i| = 0\), which defines a perpendicular bisector of the segment joining two points on the imaginary axis.
Circle Equation
Circles play a vital role in understanding geometric shapes using complex numbers. In the equation \(|z|-2=0\), the modulus \(|z|\) represents the distance of a point from the origin. Therefore, \(|z| = 2\) denotes a circle centered at the origin with a radius of 2.
- Given a complex number \(z = x + yi\), the modulus \(|z|\) is calculated as \(\sqrt{x^2 + y^2}\).
- Thus, the circle's equation can be written as \(x^2 + y^2 = 4\), where 4 is the square of the radius.
Perpendicular Bisector
To understand the concept of a **perpendicular bisector** in the complex plane, think of it as an invisible line that perfectly divides a segment into two equal parts, while being perpendicular to it. This is key when analyzing the equation \(|z-i|-|z+5i|=0\).
- This equation describes a situation where any point \(z\) has an equal distance from both the points \(0,1\) (or \(i\)) and \(0,-5\) (or \(5i\)).
- The perpendicular bisector of a segment connecting these points is derived by calculating their midpoint, here yielding the line \(y = -2\).
Complex Plane
The **complex plane** is a two-dimensional plane where each point corresponds to a complex number. With the horizontal axis representing real numbers and the vertical axis for the imaginary part, it is a major tool in visualizing and solving complex number problems.
- A complex number \(z = x + yi\) places the real part \(x\) along the x-axis and the imaginary part \(y\) along the y-axis.
- This plane becomes particularly useful when dealing with movements and transformations, as shown in our exercise's intersection problem.