Solve the equation z5+32=0z^{5}+32=0, expressing your answers in the form reiθre^{i\theta}, where r>0r>0 and −π<θ≤π-\pi<\theta\le\pi.

  1. z1z_{1}, z2z_{2} and z3z_{3} are three of the roots of z5+32=0z^{5}+32=0 such that

    0<arg⁡z1<arg⁡z2<arg⁡z3≤π .0<\arg z_{1}<\arg z_{2}<\arg z_{3}\le\pi\,.

    1. (i)

      Find the smallest positive integer nn such that (z1z2)n\left(\dfrac{z_{1}}{z_{2}}\right)^{n} is real and positive.

    2. (ii)

      The points AA and BB represent the roots z1z_{1} and z3z_{3} respectively in the Argand diagram. The line segment BA′BA' is obtained by rotating the line segment BABA through π2\dfrac{\pi}{2} clockwise about the point BB. Find the real part of the complex number represented by point A′A', giving your answer in exact trigonometric form.

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Worked solution

2[0](i)

1. Identify the roots $z_1$ and $z_2$ from the equation $z^5+32=0$.

z5=−32=32eiπz^5 = -32 = 32e^{i\pi}
z=2eiπ+2kπ5for k=−2,−1,0,1,2z = 2e^{i\frac{\pi + 2k\pi}{5}} \quad \text{for } k = -2, -1, 0, 1, 2
Considering the roots with arguments in (0,π](0, \pi]:
For k=0k=0, arg⁡z=π5\arg z = \frac{\pi}{5}
For k=1k=1, arg⁡z=3π5\arg z = \frac{3\pi}{5}
For k=2k=2, arg⁡z=π\arg z = \pi
Since 0<arg⁡z1<arg⁡z2<arg⁡z3≤π0 < \arg z_1 < \arg z_2 < \arg z_3 \le \pi, we can deduce:
z1=2eiπ5andz2=2ei3π5z_1 = 2e^{i\frac{\pi}{5}} \quad \text{and} \quad z_2 = 2e^{i\frac{3\pi}{5}}

2. Find the ratio $\frac{z_1}{z_2}$ in exponential form.

z1z2=2eiπ52ei3π5\frac{z_1}{z_2} = \frac{2e^{i\frac{\pi}{5}}}{2e^{i\frac{3\pi}{5}}}
=ei(π5−3π5)= e^{i\left(\frac{\pi}{5} - \frac{3\pi}{5}\right)}
=e−i2π5= e^{-i\frac{2\pi}{5}}

3. Apply the condition for $\left(\frac{z_1}{z_2}\right)^n$ to be real and positive to find the smallest positive integer $n$.

(z1z2)n=(e−i2π5)n=e−i2nπ5\left(\frac{z_1}{z_2}\right)^n = \left(e^{-i\frac{2\pi}{5}}\right)^n = e^{-i\frac{2n\pi}{5}}
For a complex number to be real and positive, its argument must be an integer multiple of 2π2\pi:
−2nπ5=2mπfor some integer m-\frac{2n\pi}{5} = 2m\pi \quad \text{for some integer } m
n=−5mn = -5m
To find the smallest positive integer nn, we substitute m=−1m = -1:
n=5n = 5

Answer

n=5n = 5

2[0](ii)

1. Identify the complex numbers $z_1$ and $z_3$ from the roots of $z^5+32=0$.

z5=−32=32eiπz^5 = -32 = 32e^{i\pi}
The roots are z=2ei(π+2kπ5)z = 2e^{i\left(\frac{\pi+2k\pi}{5}\right)}.
For roots with 0<arg⁡z≤π0 < \arg z \le \pi:
k=0  ⟹  z=2eiπ5k=0 \implies z = 2e^{i\frac{\pi}{5}}
k=1  ⟹  z=2ei3π5k=1 \implies z = 2e^{i\frac{3\pi}{5}}
k=2  ⟹  z=2eiπ=−2k=2 \implies z = 2e^{i\pi} = -2
Given 0<arg⁡z1<arg⁡z2<arg⁡z3≤π0 < \arg z_1 < \arg z_2 < \arg z_3 \le \pi, we have z1=2eiπ5z_1 = 2e^{i\frac{\pi}{5}} and z3=−2z_3 = -2.

2. Formulate the relationship for a clockwise rotation by $\frac{\pi}{2}$ about point $B$.

A clockwise rotation of π2\frac{\pi}{2} corresponds to multiplication by e−iπ2=−ie^{-i\frac{\pi}{2}} = -i.
The vector BA⃗\vec{BA} is represented by z1−z3z_1 - z_3, and BA′⃗\vec{BA'} is represented by zA′−z3z_{A'} - z_3.
Therefore,
zA′−z3=−i(z1−z3)z_{A'} - z_3 = -i(z_1 - z_3)

3. Substitute $z_1$ and $z_3$ and convert to Cartesian form to solve for $z_{A'}$.

zA′−(−2)=−i(2eiπ5−(−2))z_{A'} - (-2) = -i\left( 2e^{i\frac{\pi}{5}} - (-2) \right)
zA′+2=−i(2cos⁡π5+2isin⁡π5+2)z_{A'} + 2 = -i\left( 2\cos\frac{\pi}{5} + 2i\sin\frac{\pi}{5} + 2 \right)
zA′+2=−2icos⁡π5−2i2sin⁡π5−2iz_{A'} + 2 = -2i\cos\frac{\pi}{5} - 2i^2\sin\frac{\pi}{5} - 2i
Using i2=−1i^2 = -1:
zA′+2=2sin⁡π5−i(2cos⁡π5+2)z_{A'} + 2 = 2\sin\frac{\pi}{5} - i\left(2\cos\frac{\pi}{5} + 2\right)
zA′=−2+2sin⁡π5−i(2cos⁡π5+2)z_{A'} = -2 + 2\sin\frac{\pi}{5} - i\left(2\cos\frac{\pi}{5} + 2\right)

4. Extract the real part of $z_{A'}$.

The real part of zA′z_{A'} is the component without the ii:
Re⁡(zA′)=−2+2sin⁡π5\operatorname{Re}(z_{A'}) = -2 + 2\sin\frac{\pi}{5}

Answer

−2+2sin⁡(π5)-2 + 2\sin\left(\frac{\pi}{5}\right)

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