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
Analyzing the equation |z| - 2 = 0
The equation \(|z| - 2 = 0\) means that the modulus of the complex number \(z\) is 2. Therefore, the locus described by this equation is a circle centered at the origin with a radius of 2.
02
Analyzing the equation |z-i| - |z+5i| = 0
The equation \(|z - i| - |z + 5i| = 0\) can be rewritten as \(|z - i| = |z + 5i|\). This describes the locus of points equidistant from the complex numbers \(i\) and \(-5i\). Such a locus is a perpendicular bisector of the line segment joining \(i (0,1)\) and \(-5i (0,-5)\) in the complex plane.
03
Determine the perpendicular bisector
To find the perpendicular bisector of the points \((0, 1)\) and \((0, -5)\), we first find the midpoint of the segment joining these two points: \((0, (1 + (-5))/2) = (0, -2)\). The perpendicular bisector is a horizontal line passing through this point, \(y = -2\).
04
Intersecting the loci
The intersection of the circle \(|z| = 2\) and the line \(y = -2\) must be found. The points on the circle have coordinates \((x, y)\) such that \(x^2 + y^2 = 4\). Setting \(y = -2\) gives \(x^2 + (-2)^2 = 4\), or \(x^2 + 4 = 4\), leading to \(x^2 = 0\), so \(x = 0\). Thus, the point \((0, -2)\) is the sole solution.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Locus
The term "locus" signifies the set of points satisfying a particular condition or a collection of rules. In complex numbers, the locus is a collection of points in the complex plane that these numbers correspond to. If you're working with an algebraic equation that involves complex numbers, the solution often describes a geometric shape or a path, known as the locus.
In our given exercise, there are two main equations that describe loci:
In our given exercise, there are two main equations that describe loci:
- The first equation \(|z| - 2 = 0\) represents a circle's circumference centered at the origin.
- The second equation \(|z-i| - |z+5i| = 0\) describes a line on the complex plane that is equidistant from the points corresponding to \(+i\) and \(-5i\).
Modulus
In the context of complex numbers, the modulus of a complex number is its distance from the origin of the complex plane. If a point is labeled as \(z = x + yi\), where \(x\) is the real part and \(y\) is the imaginary part, the modulus is represented as \(|z| = \sqrt{x^2 + y^2}\).
- The modulus essentially converts a complex number into a positive real number, showing its magnitude in the complex plane.
- In our example, \(|z| - 2 = 0\), suggests that the distance (modulus) of complex number \(z\) from the origin is \(2\), characterizing a circle around the origin.
Perpendicular Bisector
A perpendicular bisector, in its simplest form, is a line that divides another line into two equal parts at a 90-degree angle. When dealing with complex numbers, we often see perpendicular bisectors show up as a result of equations that state one point is equidistant from two fixed points.
For the equation \(|z - i| = |z + 5i|\), think of it as a scenario where each point \(z\) on this line is equidistant to \(i (0, 1)\) and \(-5i (0, -5)\). Here, the perpendicular bisector runs horizontally through the midpoint calculated as \((0, -2)\).
For the equation \(|z - i| = |z + 5i|\), think of it as a scenario where each point \(z\) on this line is equidistant to \(i (0, 1)\) and \(-5i (0, -5)\). Here, the perpendicular bisector runs horizontally through the midpoint calculated as \((0, -2)\).
- The presence of a perpendicular bisector indicates symmetry on the complex plane, reflecting equal distances from two points.
- It's crucial for geometrically dividing sections of circles, such as in circle intersections.
Circle Equation
The classic circle equation in the complex plane is derived from the general equation \(x^2 + y^2 = r^2\), where \(r\) is the radius, and the center lies on the origin, represented by the equation \(|z| = r\). Circles described in this manner are quite common in complex numbers problems, showcasing loci centered around specific points.
In our example from the exercise, \(|z| - 2 = 0\) means that every point \(z\) on the circle is \(2\) units away from the origin.
In our example from the exercise, \(|z| - 2 = 0\) means that every point \(z\) on the circle is \(2\) units away from the origin.
- Such equations help easily identify circles in the complex plane, providing the benefit of recognizing and calculating intersections with other lines, such as that of a perpendicular bisector.
- Understanding circle equations is fundamental to grasp geometric implications of algebraic conditions on complex numbers.