What is the difference between the phase and argument of a complex number?

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SUMMARY

The discussion clarifies the distinction between the phase and argument of complex numbers, specifically addressing purely imaginary numbers. The phase of a complex number is defined as the angle it makes with the positive real axis, while the argument is calculated using the arctangent function based on the coordinates of the complex number. For purely imaginary numbers like i*2π, the argument is correctly identified as π/2, as it lies on the y-axis. The confusion arises from attempting to compute arctan(2π/0), which is undefined, but the argument's definition provides a clear resolution.

PREREQUISITES
  • Understanding of complex numbers in Cartesian form (a + ib)
  • Familiarity with the arctangent function and its properties
  • Knowledge of the unit circle and angles in radians
  • Basic concepts of trigonometry related to angles and quadrants
NEXT STEPS
  • Study the properties of complex numbers and their representations
  • Learn about the polar form of complex numbers and how to convert between forms
  • Explore the concept of the Arg function in complex analysis
  • Investigate the geometric interpretation of complex numbers on the complex plane
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Mathematics students, educators, and anyone interested in understanding complex analysis and the geometric representation of complex numbers.

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in cartesian form, a+ ib you can find the phase by doing arctan(b/a).. my question concerns the phase of a purely imaginary number. during a lecture my professor said that the phase of i*2pi= pi/2, he rationalized this by saying that the number lies on the y-axis so the angle between the real axis and the imaginary axis is pi/2. but if you do arctan(2pi/0) you will get an error.. how is he right?
 
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Complex numbers don't have a "phase" they have an "argument" which is defined case by case: if z = x + iy we define \phi = Arg(z) as
arctan(y/x) when x > 0
arctan(y/x)+π when x < 0 and y ≥ 0
arctan(y/x)-π when x < 0 and y < 0
π/2 when x = 0 and y > 0
-π/2 when x = 0 and y < 0
indeterminate when x = 0 and y = 0.
 

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