The complex number zz satisfies ∣z−4−3i∣≤2andπ<arg⁡[(z−4)2]<2π|z-4-3\text{i}| \le 2 \quad \text{and} \quad \pi < \arg\left[(z-4)^2\right] < 2\pi

  1. (i)

    Sketch clearly the locus of zz on an Argand diagram.

  2. (ii)

    Find the range of values of ∣z−8∣|z-8|.

  3. (iii)

    Find maximum value of arg⁡(z−8)\arg(z-8).

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Worked solution

3(i)

1. Identify the region defined by the first inequality.

The inequality ∣z−4−3i∣≤2|z - 4 - 3\text{i}| \le 2 can be rewritten as:
∣z−(4+3i)∣≤2|z - (4 + 3\text{i})| \le 2
This represents the set of points inside and on the boundary of a solid circular disk with centre C(4,3)C(4, 3) and radius 22.

2. Determine the geometric constraint imposed by the argument inequality.

Let θ=arg⁡(z−4)\theta = \arg(z-4). Then arg⁡[(z−4)2]=2θ\arg\left[(z-4)^2\right] = 2\theta (modulo 2π2\pi).
For the condition π<arg⁡[(z−4)2]<2π\pi < \arg\left[(z-4)^2\right] < 2\pi to hold, the complex number (z−4)2(z-4)^2 must lie strictly in the lower half-plane.
This means 2θ2\theta corresponds to an angle in the third or fourth quadrant, which implies θ\theta must lie in the second or fourth quadrant:
θ∈(π2,π)∪(−π2,0)\theta \in \left(\frac{\pi}{2}, \pi\right) \cup \left(-\frac{\pi}{2}, 0\right)

3. Combine both conditions to define the exact boundary of the locus.

For points in the disk ∣z−(4+3i)∣≤2|z - (4+3\text{i})| \le 2, the imaginary part of zz ranges from 3−2=13 - 2 = 1 to 3+2=53 + 2 = 5.
Since Im(z)≥1>0\text{Im}(z) \ge 1 > 0, the vector representing z−4z-4 must point upwards, meaning its angle θ\theta must lie in the upper half-plane, i.e., (0,π)(0, \pi).
Intersecting this with the quadrants found in Step 2, θ\theta must strictly be in the second quadrant:
θ∈(π2,π)\theta \in \left(\frac{\pi}{2}, \pi\right)
This strictly requires the real part of z−4z-4 to be negative:
Re(z−4)<0  ⟹  Re(z)<4\text{Re}(z-4) < 0 \implies \text{Re}(z) < 4
Thus, the required region is the strict left half of the circular disk.

Answer

Sketch Description:

  • Draw the real (Re\text{Re}) and imaginary (Im\text{Im}) axes.
  • Identify the circle centred at C(4,3)C(4, 3) with radius 22.
  • Draw a dotted vertical line segment along x=4x = 4 between the points (4,1)(4, 1) and (4,5)(4, 5).
  • Draw a solid semi-circular arc passing through (4,5)(4, 5), (2,3)(2, 3), and (4,1)(4, 1), representing the left half of the circle.
  • Shade the region enclosed by the dotted line segment and the solid arc.
  • Label key coordinates: centre (4,3)(4, 3), and boundary points (2,3)(2, 3), (4,1)(4, 1), and (4,5)(4, 5).

3(ii)

1. Contextualise the locus of $z$ using the conditions from part (i).

From part (i), the condition ∣z−(4+3i)∣≤2|z - (4+3\text{i})| \le 2 represents a solid disk DD centred at C(4,3)C(4,3) with radius 2.
For any point in DD, y≥1>0y \ge 1 > 0, so arg⁡(z−4)\arg(z-4) naturally lies in the interval (0,π)(0, \pi).
The argument condition π<arg⁡[(z−4)2]<2π\pi < \arg[(z-4)^2] < 2\pi implies:
π<2arg⁡(z−4)<2π  ⟹  π2<arg⁡(z−4)<π\pi < 2\arg(z-4) < 2\pi \implies \frac{\pi}{2} < \arg(z-4) < \pi
This strictly limits z−4z-4 to the second quadrant, meaning Re(z−4)<0  ⟹  x<4\text{Re}(z-4) < 0 \implies x < 4.
Thus, the locus of zz is the left-half of DD, excluding the vertical boundary x=4x=4.

2. Calculate the maximum possible value of $|z-8|$.

∣z−8∣|z-8| represents the distance from zz to the point A(8,0)A(8,0).
The distance from AA to the centre C(4,3)C(4,3) is:
∣C−A∣=∣(4+3i)−8∣=∣−4+3i∣=(−4)2+32=5|C - A| = |(4+3\text{i}) - 8| = |-4+3\text{i}| = \sqrt{(-4)^2 + 3^2} = 5
The maximum possible distance from AA to the full disk DD is 5+2=75 + 2 = 7.
The point achieving this maximum distance is directly opposite AA through CC:
zmax=A+(5+25)(C−A)=8+75(−4+3i)=2.4+4.2iz_{\text{max}} = A + \left(\frac{5+2}{5}\right)(C - A) = 8 + \frac{7}{5}(-4+3\text{i}) = 2.4 + 4.2\text{i}
Since Re(zmax)=2.4<4\text{Re}(z_{\text{max}}) = 2.4 < 4, zmaxz_{\text{max}} lies well inside our restricted locus. Thus, the maximum value of ∣z−8∣|z-8| is exactly 77.

3. Determine the infimum (strict minimum limit) of $|z-8|$ constrained by the locus.

The minimum possible distance from AA to the full disk DD is 5−2=35 - 2 = 3, which would occur at:
zmin=8+35(−4+3i)=5.6+1.8iz_{\text{min}} = 8 + \frac{3}{5}(-4+3\text{i}) = 5.6 + 1.8\text{i}
Since Re(zmin)=5.6≥4\text{Re}(z_{\text{min}}) = 5.6 \ge 4, this unconstrained minimum is outside the locus.
For points restricted to the locus, the square of the distance to A(8,0)A(8,0) is (x−8)2+y2(x-8)^2 + y^2.
Because x<4x < 4, we have x−8<−4  ⟹  (x−8)2>16x-8 < -4 \implies (x-8)^2 > 16.
Additionally, for all points inside the disk DD, the minimum possible value for yy is 11.
As zz gets arbitrarily close to 4+i4+\text{i} (the bottom-right limiting boundary of the locus), the distance approaches:
(4−8)2+12=16+1=17\sqrt{(4-8)^2 + 1^2} = \sqrt{16 + 1} = \sqrt{17}
Because the boundary x=4x=4 is strictly excluded, ∣z−8∣|z-8| must be strictly greater than 17\sqrt{17}.

4. State the final range of values.

Combining the strict lower bound and the achieved maximum distance, the range of values is:
17<∣z−8∣≤7\sqrt{17} < |z-8| \le 7

Answer

17<∣z−8∣≤7\sqrt{17} < |z-8| \le 7

3(iii)

1. Identify that the maximum argument corresponds to the tangent from $z=8$ to the circular locus.

The locus of zz is a region within the circle ∣z−(4+3i)∣≤2|z-(4+3\text{i})| \le 2. The expression arg⁡(z−8)\arg(z-8) represents the angle made by the vector from P(8,0)P(8,0) to a point zz in the locus with the positive real axis.
To maximise this angle, we must consider the tangent line from P(8,0)P(8,0) to the circle that gives the largest angle, touching the circle at a point TT.

2. Calculate the relevant angles in the right-angled triangle formed by the centre of the circle, the point $z=8$, and the point of tangency.

Let C(4,3)C(4,3) be the centre of the circle. In the right-angled △PTC\triangle PTC, the distance from P(8,0)P(8,0) to C(4,3)C(4,3) is:
PC=∣4+3i−8∣=∣−4+3i∣=(−4)2+32=5PC = |4+3\text{i} - 8| = |-4+3\text{i}| = \sqrt{(-4)^2 + 3^2} = 5
The radius of the circle is CT=2CT = 2.
The angle ∠CPT\angle CPT between the line PCPC and the tangent line PTPT is given by:
sin⁡(∠CPT)=CTPC=25  ⟹  ∠CPT=arcsin⁡(25)≈0.4115 rad\sin(\angle CPT) = \frac{CT}{PC} = \frac{2}{5} \implies \angle CPT = \arcsin\left(\frac{2}{5}\right) \approx 0.4115 \text{ rad}

3. Find the angle of the line $PC$ and combine it with $\angle CPT$ to find the maximum argument.

The angle of the vector PC⃗=−4+3i\vec{PC} = -4+3\text{i} from the positive real axis is:
π−arctan⁡(34)\pi - \arctan\left(\frac{3}{4}\right)
To maximise the argument, the tangent PTPT must be rotated further counter-clockwise from PCPC relative to PP. Thus, we add ∠CPT\angle CPT to the angle of PCPC:
max⁡arg⁡(z−8)=π−arctan⁡(34)+∠CPT\max \arg(z-8) = \pi - \arctan\left(\frac{3}{4}\right) + \angle CPT
max⁡arg⁡(z−8)=π−arctan⁡(34)+arcsin⁡(25)\max \arg(z-8) = \pi - \arctan\left(\frac{3}{4}\right) + \arcsin\left(\frac{2}{5}\right)
max⁡arg⁡(z−8)≈π−0.6435+0.4115≈2.91 rad\max \arg(z-8) \approx \pi - 0.6435 + 0.4115 \approx 2.91 \text{ rad}
(Note: The point of tangency TT has x≈3.54<4x \approx 3.54 < 4, which strictly satisfies the locus condition π<arg⁡[(z−4)2]<2π\pi < \arg\left[(z-4)^2\right] < 2\pi.)

Answer

2.91 rad2.91 \text{ rad}

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