Solve the equation iz5=−32iz^5=-32, giving your roots in the form reiθre^{i\theta}, where r>0r>0 and −π<θ≤π-\pi<\theta\le\pi. Sketch on an Argand diagram the points P1,P2,P3,P4P_1, P_2, P_3, P_4 and P5P_5 representing these roots, where P1P_1 represents the root with the smallest argument and P1  P2  P3  P4  P5P_1\;P_2\;P_3\;P_4\;P_5 is a polygon described in an anticlockwise sense. Find the area of P1  P2  P3  P4  P5P_1\;P_2\;P_3\;P_4\;P_5. On an Argand diagram sketch clearly the locus of PP where PP represents the complex number zz such that ∣z−2−2i∣≤1|z-2-2i|\le 1 and arg⁡(z−1)=arg⁡(1+3i)\arg(z-1)=\arg(1+3i). Find the range of values of arg⁡(z−3−2i)\arg(z-3-2i), given that ∣z−2−2i∣≤1|z-2-2i|\le 1 and arg⁡(z−1)=arg⁡(1+3i)\arg(z-1)=\arg(1+3i).

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Worked solution

8(a)

1. Rearrange the equation and express the right-hand side in exponential form.

iz5=−32  ⟹  z5=−32i=32iiz^5 = -32 \implies z^5 = \frac{-32}{i} = 32i
32i=32eiπ232i = 32e^{i\frac{\pi}{2}}

2. Formulate the general expression for the roots.

z=(32ei(π2+2kπ))15=2ei(π10+2kπ5),k∈Zz = \left( 32e^{i\left(\frac{\pi}{2} + 2k\pi\right)} \right)^{\frac{1}{5}} = 2e^{i\left(\frac{\pi}{10} + \frac{2k\pi}{5}\right)}, \quad k \in \mathbb{Z}

3. Find the 5 specific roots in the required principal argument range.

Evaluating for k=−2,−1,0,1,2k = -2, -1, 0, 1, 2 to keep −π<θ≤π-\pi < \theta \le \pi:
k=−2  ⟹  z=2e−i7π10k = -2 \implies z = 2e^{-i\frac{7\pi}{10}}
k=−1  ⟹  z=2e−i3π10k = -1 \implies z = 2e^{-i\frac{3\pi}{10}}
k=0  ⟹  z=2eiπ10k = 0 \implies z = 2e^{i\frac{\pi}{10}}
k=1  ⟹  z=2eiπ2k = 1 \implies z = 2e^{i\frac{\pi}{2}}
k=2  ⟹  z=2ei9π10k = 2 \implies z = 2e^{i\frac{9\pi}{10}}

4. Sketch the Argand diagram and identify points $P_1$ to $P_5$.

The 5 points lie on a circle of radius 2 centered at the origin, separated by angles of 2π5\frac{2\pi}{5}.
P1P_1 has the smallest argument, so P1=2e−i7π10P_1 = 2e^{-i\frac{7\pi}{10}} (in the 3rd quadrant).
Following an anticlockwise direction:
P2=2e−i3π10P_2 = 2e^{-i\frac{3\pi}{10}} (in the 4th quadrant)
P3=2eiπ10P_3 = 2e^{i\frac{\pi}{10}} (in the 1st quadrant)
P4=2eiπ2P_4 = 2e^{i\frac{\pi}{2}} (on the positive imaginary axis)
P5=2ei9π10P_5 = 2e^{i\frac{9\pi}{10}} (in the 2nd quadrant)
(A regular pentagon is sketched on the Argand diagram with vertices at these points)

5. Calculate the area of the polygon $P_1P_2P_3P_4P_5$.

The regular pentagon can be divided into 5 identical isosceles triangles radiating from the origin. Each triangle has side lengths of 2 and an included angle of 2π5\frac{2\pi}{5}.
Area=5×(12(2)(2)sin⁡(2π5))\text{Area} = 5 \times \left( \frac{1}{2}(2)(2)\sin\left(\frac{2\pi}{5}\right) \right)
Area=10sin⁡(2π5)≈9.51 units2\text{Area} = 10\sin\left(\frac{2\pi}{5}\right) \approx 9.51 \text{ units}^2

Answer

Roots: z=2e−i7π10,2e−i3π10,2eiπ10,2eiπ2,2ei9π10z = 2e^{-i\frac{7\pi}{10}}, 2e^{-i\frac{3\pi}{10}}, 2e^{i\frac{\pi}{10}}, 2e^{i\frac{\pi}{2}}, 2e^{i\frac{9\pi}{10}}.
Area = 10sin⁡(2π5)10\sin\left(\frac{2\pi}{5}\right) or 9.519.51 units2^2.

8(b)

1. Identify the two loci and formulate equations to find their intersection.

The inequality ∣z−(2+2i)∣≤1|z - (2+2i)| \le 1 represents the region on and inside a circle centred at 2+2i2+2i with radius 11.

The equation arg⁡(z−1)=arg⁡(1+3i)\arg(z-1) = \arg(1+3i) represents a half-line (ray) starting from the point (1,0)(1,0) with a direction vector corresponding to 1+3i1+3i. This ray lies on the line passing through (1,0)(1,0) with gradient 33, so its Cartesian equation is y=3(x−1)y = 3(x-1) for x>1x > 1.

To sketch the locus, we must find the intersection of this ray with the boundary of the circle, (x−2)2+(y−2)2=1(x-2)^2 + (y-2)^2 = 1.

2. Solve for the endpoints of the line segment representing the locus of $P$.

Substitute y=3x−3y = 3x - 3 into the circle's boundary equation:
(x−2)2+(3x−5)2=1(x-2)^2 + (3x-5)^2 = 1
x2−4x+4+9x2−30x+25=1x^2 - 4x + 4 + 9x^2 - 30x + 25 = 1
10x2−34x+28=010x^2 - 34x + 28 = 0
5x2−17x+14=05x^2 - 17x + 14 = 0
(5x−7)(x−2)=0(5x-7)(x-2) = 0

Solving gives x=1.4x = 1.4 or x=2x = 2.
When x=1.4x = 1.4, y=3(1.4)−3=1.2y = 3(1.4) - 3 = 1.2.
When x=2x = 2, y=3(2)−3=3y = 3(2) - 3 = 3.
Both points satisfy x>1x > 1 and y>0y > 0, so they lie on the half-line.
The locus of PP is the line segment connecting 1.4+1.2i1.4+1.2i and 2+3i2+3i. (The sketch should show this specific line segment bounded within the circle).

3. Evaluate the argument of $z-3-2i$ at the endpoints of the locus.

Let w=z−(3+2i)w = z - (3+2i). We need the range of arg⁡(w)\arg(w) as zz varies along the segment.

At the endpoint z1=2+3iz_1 = 2+3i:
w1=(2+3i)−(3+2i)=−1+iw_1 = (2+3i) - (3+2i) = -1 + i
arg⁡(w1)=π−tan⁡−1(1)=3π4\arg(w_1) = \pi - \tan^{-1}(1) = \frac{3\pi}{4}

At the endpoint z2=1.4+1.2iz_2 = 1.4+1.2i:
w2=(1.4+1.2i)−(3+2i)=−1.6−0.8iw_2 = (1.4+1.2i) - (3+2i) = -1.6 - 0.8i
Since this point is in the third quadrant:
arg⁡(w2)=−π+tan⁡−1(0.81.6)=−π+tan⁡−1(12)\arg(w_2) = -\pi + \tan^{-1}\left(\frac{0.8}{1.6}\right) = -\pi + \tan^{-1}\left(\frac{1}{2}\right)

4. Determine the full range of $\arg(z-3-2i)$ by checking the transition of the angle along the segment.

As zz moves along the segment from 2+3i2+3i to 1.4+1.2i1.4+1.2i, the vector ww moves along a straight line from −1+i-1+i to −1.6−0.8i-1.6-0.8i.

This path crosses the negative real axis when the imaginary part of ww is zero. This occurs at y=2y = 2.
On the line y=3x−3y = 3x - 3, when y=2y = 2, x=53x = \frac{5}{3}.
At this intercept, w=(53−3)+0i=−43w = \left(\frac{5}{3}-3\right) + 0i = -\frac{4}{3}, which has an argument of exactly π\pi.

Thus, the argument monotonically increases from 3π4\frac{3\pi}{4} up to π\pi in the second quadrant. Upon crossing the negative real axis, the principal argument wraps to −π-\pi and increases to −π+tan⁡−1(12)-\pi + \tan^{-1}\left(\frac{1}{2}\right) in the third quadrant.

The range of values is therefore:
3π4≤arg⁡(z−3−2i)≤π\frac{3\pi}{4} \le \arg(z-3-2i) \le \pi or −π<arg⁡(z−3−2i)≤−π+tan⁡−1(12)-\pi < \arg(z-3-2i) \le -\pi + \tan^{-1}\left(\frac{1}{2}\right)

Answer

3π4≤arg⁡(z−3−2i)≤π\frac{3\pi}{4} \le \arg(z-3-2i) \le \pi or −π<arg⁡(z−3−2i)≤−π+tan⁡−1(12)-\pi < \arg(z-3-2i) \le -\pi + \tan^{-1}\left(\frac{1}{2}\right)

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