复数 zz 满足以下不等式:
∣z∣2≤4and−π6≤arg⁡(z+3−i)≤0.|z|^2 \le 4 \quad \text{and} \quad -\frac{\pi}{6} \le \arg(z + \sqrt{3} - \mathrm{i}) \le 0.

  1. (i)

    在阿甘德图上,画出代表 zz 的点可能所在的区域 RR。

  2. (ii)

    求 ∣z−2i∣|z - 2\mathrm{i}| 的最大值 and 最小值的精确值。

  3. (iii)

    确定方程 z100=2100z^{100} = 2^{100} 的根落在区域 RR 内的个数。

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完整解法

8(i)

1. Interpret the modulus inequality $|z|^2 \le 4$.

∣z∣2≤4  ⟹  ∣z∣≤2|z|^2 \le 4 \implies |z| \le 2
This defines the region inside and on the boundary of a circle centred at the origin (0,0)(0,0) with radius 22.

2. Interpret the argument inequality.

−π6≤arg⁡(z−(−3+i))≤0-\frac{\pi}{6} \le \arg(z - (-\sqrt{3} + \mathrm{i})) \le 0
This defines a region bounded by two half-lines emanating from the point A(−3,1)A(-\sqrt{3}, 1).
The first half-line has an angle of 00 (horizontal, extending to the right).
The second half-line has an angle of −π6-\frac{\pi}{6} (directed downwards to the right).

3. Determine the position of the starting point and the intersections of the half-lines with the circle.

The modulus of A(−3,1)A(-\sqrt{3}, 1) is (−3)2+12=2\sqrt{(-\sqrt{3})^2 + 1^2} = 2, so AA lies exactly on the circle ∣z∣=2|z| = 2.
The horizontal half-line y=1y=1 (for x≥−3x \ge -\sqrt{3}) intersects x2+y2=4x^2+y^2=4 at x2+1=4  ⟹  x=3x^2+1=4 \implies x=\sqrt{3}. This gives the intersection point (3,1)(\sqrt{3}, 1).
The half-line at angle −π6-\frac{\pi}{6} has gradient tan⁡(−π6)=−13\tan(-\frac{\pi}{6}) = -\frac{1}{\sqrt{3}}.
Its equation is y−1=−13(x+3)  ⟹  y=−x3y - 1 = -\frac{1}{\sqrt{3}}(x + \sqrt{3}) \implies y = -\frac{x}{\sqrt{3}}.
This line passes through the origin (0,0)(0,0) and intersects the circle when x2+x23=4  ⟹  x2=3x^2 + \frac{x^2}{3} = 4 \implies x^2 = 3.
Since it extends to the right, x=3x = \sqrt{3}, which gives y=−1y = -1. This corresponds to the point (3,−1)(\sqrt{3}, -1).

4. Describe the required sketch of region $R$.

The sketch must include:

  • A circle (or arc) centred at OO with radius 22.
  • The starting point marked at −3+i-\sqrt{3} + \mathrm{i}.
  • A horizontal half-line from −3+i-\sqrt{3} + \mathrm{i} ending at 3+i\sqrt{3} + \mathrm{i}.
  • A half-line from −3+i-\sqrt{3} + \mathrm{i} passing through the origin OO and ending at 3−i\sqrt{3} - \mathrm{i}.
  • The region RR shaded, bounded by these two segments and the minor arc of the circle between 3+i\sqrt{3} + \mathrm{i} and 3−i\sqrt{3} - \mathrm{i}.
答案

A valid Argand diagram showing the region bounded by two line segments from −3+i-\sqrt{3}+\mathrm{i} to 3+i\sqrt{3}+\mathrm{i} and 3−i\sqrt{3}-\mathrm{i}, and the minor arc of the circle ∣z∣=2|z|=2 connecting 3+i\sqrt{3}+\mathrm{i} to 3−i\sqrt{3}-\mathrm{i}.

8(ii)

1. Interpret $|z - 2\mathrm{i}|$ geometrically and find the minimum value by identifying the closest point in region $R$.

Let z=x+iyz = x + \mathrm{i}y. The expression ∣z−2i∣|z - 2\mathrm{i}| represents the distance from the point (0,2)(0, 2) to a point zz in region RR.
From the inequalities defining RR, the upper boundary is given by arg⁡(z+3−i)=0\arg(z + \sqrt{3} - \mathrm{i}) = 0, which is the horizontal line segment y=1y = 1 for −3≤x≤3-\sqrt{3} \le x \le \sqrt{3}.
The perpendicular distance from (0,2)(0, 2) to the line y=1y = 1 is 2−1=12 - 1 = 1.
Since the foot of the perpendicular, (0,1)(0, 1), lies on this boundary segment (as −3≤0≤3-\sqrt{3} \le 0 \le \sqrt{3}), it is the closest point in RR.
Thus, the minimum possible value of ∣z−2i∣|z - 2\mathrm{i}| is exactly 11.

2. Set up an algebraic expression for the distance squared to evaluate the maximum value.

To find the maximum distance, we can express the square of the distance algebraically:
∣z−2i∣2=x2+(y−2)2|z - 2\mathrm{i}|^2 = x^2 + (y - 2)^2
∣z−2i∣2=x2+y2−4y+4|z - 2\mathrm{i}|^2 = x^2 + y^2 - 4y + 4
For any point zz in the region RR, it must lie within the disc ∣z∣2≤4|z|^2 \le 4, so we know that x2+y2≤4x^2 + y^2 \le 4.

3. Determine the lowest point in $R$ to bound $y$, and hence calculate the maximum distance.

The maximum of ∣z−2i∣2|z - 2\mathrm{i}|^2 will occur when yy is as small as possible.
The lowest point in RR is the intersection of the lower ray arg⁡(z+3−i)=−π6\arg(z + \sqrt{3} - \mathrm{i}) = -\frac{\pi}{6} and the circle x2+y2=4x^2 + y^2 = 4.
The ray passes through (−3,1)(-\sqrt{3}, 1) with gradient tan⁡(−π6)=−13\tan(-\frac{\pi}{6}) = -\frac{1}{\sqrt{3}}, giving the line x+3y=0x + \sqrt{3}y = 0.
Substituting x=−3yx = -\sqrt{3}y into x2+y2=4x^2 + y^2 = 4 yields 3y2+y2=4  ⟹  y2=13y^2 + y^2 = 4 \implies y^2 = 1.
Since the ray extends in the positive real direction, the intersection is at y=−1y = -1 (the point S(3,−1)S(\sqrt{3}, -1)).
Thus, for all z∈Rz \in R, we have y≥−1y \ge -1, which means −4y≤4-4y \le 4.
Substituting the geometric bounds x2+y2≤4x^2 + y^2 \le 4 and −4y≤4-4y \le 4 into our expression:
∣z−2i∣2≤4+4+4=12|z - 2\mathrm{i}|^2 \le 4 + 4 + 4 = 12
The maximum possible value of ∣z−2i∣|z - 2\mathrm{i}| is exactly 12=23\sqrt{12} = 2\sqrt{3}.

答案

Minimum value = 11
Maximum value = 232\sqrt{3}

8(iii)

1. Express the roots of the given equation in exponential form.

The equation z100=2100z^{100} = 2^{100} can be rewritten as z100=2100ei2kπz^{100} = 2^{100}e^{\mathrm{i}2k\pi} for k∈Zk \in \mathbb{Z}.
Solving for zz gives:
z=2ei2kπ100=2eikπ50z = 2e^{\mathrm{i}\frac{2k\pi}{100}} = 2e^{\mathrm{i}\frac{k\pi}{50}}
All roots have a modulus of 22, meaning they lie exactly on the circular boundary of the region ∣z∣2≤4|z|^2 \le 4.

2. Determine the range of arguments for the part of the circle $|z| = 2$ that lies in region $R$.

The region RR is bounded by the circle ∣z∣=2|z| = 2 and the two rays from z0=−3+iz_0 = -\sqrt{3} + \mathrm{i}.
For the horizontal ray arg⁡(z+3−i)=0\arg(z + \sqrt{3} - \mathrm{i}) = 0, the line is y=1y = 1 (for x≥−3x \ge -\sqrt{3}).
Substituting into x2+y2=4x^2 + y^2 = 4 gives x2+1=4  ⟹  x=3x^2 + 1 = 4 \implies x = \sqrt{3}. This gives the intersection z=3+iz = \sqrt{3} + \mathrm{i}, where arg⁡(z)=π6\arg(z) = \frac{\pi}{6}.

The ray arg⁡(z+3−i)=−π6\arg(z + \sqrt{3} - \mathrm{i}) = -\frac{\pi}{6} is a line with gradient tan⁡(−π6)=−13\tan(-\frac{\pi}{6}) = -\frac{1}{\sqrt{3}} passing through (−3,1)(-\sqrt{3}, 1), which simplifies to the line y=−13xy = -\frac{1}{\sqrt{3}}x.
Substituting into x2+y2=4x^2 + y^2 = 4 gives x2+13x2=4  ⟹  x=3x^2 + \frac{1}{3}x^2 = 4 \implies x = \sqrt{3} (since x>0x>0), giving the intersection z=3−iz = \sqrt{3} - \mathrm{i} with arg⁡(z)=−π6\arg(z) = -\frac{\pi}{6}.

Thus, any root lying in the main body of RR must be on the arc where:
−π6≤arg⁡(z)≤π6-\frac{\pi}{6} \le \arg(z) \le \frac{\pi}{6}
(Note: The vertex point −3+i-\sqrt{3} + \mathrm{i} is also in RR and on ∣z∣=2|z|=2, but its argument is 5π6\frac{5\pi}{6}, which is not of the form kπ50\frac{k\pi}{50}, so it is not a root.)

3. Solve for the possible values of $k$ to find the number of roots.

Substitute arg⁡(z)=kπ50\arg(z) = \frac{k\pi}{50} into the inequality:
−π6≤kπ50≤π6-\frac{\pi}{6} \le \frac{k\pi}{50} \le \frac{\pi}{6}
−506≤k≤506-\frac{50}{6} \le k \le \frac{50}{6}
−8.33≤k≤8.33-8.33 \le k \le 8.33

Since kk must be an integer, k∈{−8,−7,…,0,…,8}k \in \{-8, -7, \dots, 0, \dots, 8\}.
The number of valid roots is 8−(−8)+1=178 - (-8) + 1 = 17.

答案

17

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