To find the Amplitude or Argument of a complex number let us assume that, a complex number z = x + iy where x > 0 and y > 0 are real, i = √1 and x\(^{2}\) + y\(^{2}\) ≠ 0; for which the equations x = z cos θ and y = z sin θ are simultaneously satisfied then, the value of θ is called the Argument (Agr) of z or Amplitude (Amp) of z.
From the above equations x = z cos θ and y = z sin θ satisfies infinite values of θ and for any infinite values of θ is the value of Arg z. Thus, for any unique value of θ that lies in the interval  π < θ ≤ π and satisfies the above equations x = z cos θ and y = z sin θ is known as the principal value of Arg z or Amp z and it is denoted as arg z or amp z.
We know that, cos (2nπ + θ) = cos θ and sin (2nπ + θ) = sin θ (where n = 0, ±1, ±2, ±3, .............), then we get,
Amp z = 2nπ + amp z where  π < amp z ≤ π
Algorithm for finding
Argument of z = x + iy
Step I: Find the value of tan\(^{1}\) \(\frac{y}{x}\) lying between 0 and \(\frac{π}{2}\). Let it be α.
Step II: Determine in which quadrant the point M(x, y) belongs.
If M (x, y) belongs to the first quadrant, then arg (z) = α.
If M (x, y) belongs to the second quadrant, then arg (z) = π  α.
If M (x, y) belongs to the third quadrant, then arg (z) =  (π  α) or π + α
If M (x, y) belongs to the fourth quadrant, then arg (z) = α or 2π  α
Solved Examples to find the Argument or Amplitude of a complex number:
1. Find the argument of the complex number \(\frac{i}{1  i}\).
Solution:
The given complex number \(\frac{i}{1  i}\)
Now multiply the numerator and denominator by the conjugate of the denominator i.e., (1 + i), we get
\(\frac{i(1 + i)}{(1  i)(1 + i)}\)
= \(\frac{i + i^{2})}{(1  i^{2}}\)
= \(\frac{i  1}{2}\)
=  \(\frac{1}{2}\) + i ∙ \(\frac{1}{2}\)
We see that in the zplane the point z =  \(\frac{1}{2}\) + i ∙ \(\frac{1}{2}\) = (\(\frac{1}{2}\), \(\frac{1}{2}\)) lies in the second quadrant. Hence, if amp z = θ then,
tan θ = \(\frac{\frac{1}{2} }{ \frac{1}{2}}\) = 1, where \(\frac{π}{2}\) < θ ≤ π
Thus, tan θ = 1 = tan (π \(\frac{π}{4}\)) = tan \(\frac{3π}{4}\)
Therefore, required argument of \(\frac{i}{1  i}\) is \(\frac{3π}{4}\).
2. Find the argument of the complex number 2 + 2√3i.
Solution:
The given complex number 2 + 2√3i
We see that in the zplane the point z = 2 + 2√3i = (2, 2√3) lies in the first quadrant. Hence, if amp z = θ then,
tan θ = \(\frac{2√3 }{2}\) = √3, where θ lying between 0 and \(\frac{π}{2}\).
Thus, tan θ = √3 = tan \(\frac{π}{3}\)
Therefore, required argument of 2 + 2√3i is \(\frac{π}{3}\).
11 and 12 Grade Math
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