The polar form of a complex number zz is given by z=reiθz = r\text{e}^{\text{i}\theta}, where r>0r > 0 and 0<θ≤π0 < \theta \le \pi, and the complex number w=(13−33i)zw = \left(\frac{1}{3} - \frac{\sqrt{3}}{3}\text{i}\right)z.

  1. (i)

    Find ww in exact polar form in terms of rr and θ\theta.

  2. (ii)

    Given that z5w∗\frac{z^5}{w^*} is real and positive, find the possible value(s) of θ\theta in exact form, leaving your answer(s) in terms of π\pi.

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Worked solution

4(i)

1. Find the modulus and argument of the complex number multiplier $\frac{1}{3} - \frac{\sqrt{3}}{3}\text{i}$.

Let c=13−33ic = \frac{1}{3} - \frac{\sqrt{3}}{3}\text{i}.
∣c∣=(13)2+(−33)2=19+39=23|c| = \sqrt{\left(\frac{1}{3}\right)^2 + \left(-\frac{\sqrt{3}}{3}\right)^2} = \sqrt{\frac{1}{9} + \frac{3}{9}} = \frac{2}{3}
Since the real part is positive and the imaginary part is negative, the complex number lies in the fourth quadrant.
arg⁡(c)=tan⁡−1(−3/31/3)=tan⁡−1(−3)=−π3\arg(c) = \tan^{-1}\left(\frac{-\sqrt{3}/3}{1/3}\right) = \tan^{-1}(-\sqrt{3}) = -\frac{\pi}{3}
Thus, c=23e−iπ3c = \frac{2}{3}\text{e}^{-\text{i}\frac{\pi}{3}}.

2. Multiply the complex numbers to express $w$ in exact polar form.

w=c×z=(23e−iπ3)(reiθ)w = c \times z = \left(\frac{2}{3}\text{e}^{-\text{i}\frac{\pi}{3}}\right)\left(r\text{e}^{\text{i}\theta}\right)
w=23rei(θ−π3)w = \frac{2}{3}r\text{e}^{\text{i}\left(\theta - \frac{\pi}{3}\right)}
Since 0<θ≤π0 < \theta \le \pi, the argument θ−π3\theta - \frac{\pi}{3} lies within the principal range (−π,π](-\pi, \pi], so no adjustment is required.

Answer

w=23rei(θ−π3)w = \frac{2}{3}r\text{e}^{\text{i}\left(\theta - \frac{\pi}{3}\right)}

4(ii)

1. Express $z^5$ and $w^*$ in exponential form.

From part (i), w=23rei(θ−π3)w = \frac{2}{3}r\text{e}^{\text{i}\left(\theta - \frac{\pi}{3}\right)}.
The complex conjugate w∗w^* is found by negating the argument:
w∗=23re−i(θ−π3)=23rei(π3−θ)w^* = \frac{2}{3}r\text{e}^{-\text{i}\left(\theta - \frac{\pi}{3}\right)} = \frac{2}{3}r\text{e}^{\text{i}\left(\frac{\pi}{3} - \theta\right)}
Using standard index laws (De Moivre's Theorem for exponential forms),
z5=(reiθ)5=r5ei5θz^5 = (r\text{e}^{\text{i}\theta})^5 = r^5\text{e}^{\text{i}5\theta}

2. Find the quotient $\frac{z^5}{w^*}$ in exponential form.

z5w∗=r5ei5θ23rei(π3−θ)\frac{z^5}{w^*} = \frac{r^5\text{e}^{\text{i}5\theta}}{\frac{2}{3}r\text{e}^{\text{i}\left(\frac{\pi}{3} - \theta\right)}}
=32r4ei(5θ−(π3−θ))= \frac{3}{2}r^4 \text{e}^{\text{i}\left(5\theta - \left(\frac{\pi}{3} - \theta\right)\right)}
=32r4ei(6θ−π3)= \frac{3}{2}r^4 \text{e}^{\text{i}\left(6\theta - \frac{\pi}{3}\right)}

3. Form an equation using the condition that the quotient is real and positive.

For a complex number to be real and positive, its imaginary part is 0 and its real part is positive, which means its argument must be an integer multiple of 2π2\pi.
6θ−π3=2kπ,for k∈Z6\theta - \frac{\pi}{3} = 2k\pi, \quad \text{for } k \in \mathbb{Z}
6θ=2kπ+π3=(6k+1)π36\theta = 2k\pi + \frac{\pi}{3} = \frac{(6k + 1)\pi}{3}
θ=(6k+1)π18\theta = \frac{(6k + 1)\pi}{18}

4. Solve for the possible values of $\theta$ within the specified domain.

We are given the domain 0<θ≤π0 < \theta \le \pi.
Substituting integer values for kk:
For k=0k = 0, θ=π18\theta = \frac{\pi}{18}
For k=1k = 1, θ=7π18\theta = \frac{7\pi}{18}
For k=2k = 2, θ=13π18\theta = \frac{13\pi}{18}
For k=3k = 3, θ=19π18\theta = \frac{19\pi}{18} (Reject, as θ>π\theta > \pi)
For k<0k < 0, θ<0\theta < 0 (Reject, as θ≤0\theta \le 0)

The possible exact values are π18\frac{\pi}{18}, 7π18\frac{7\pi}{18}, and 13π18\frac{13\pi}{18}.

Answer

\frac{\pi}{18}, \frac{7\pi}{18}, \frac{13\pi}{18}

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