解方程 ,将根写成 的形式,其中 且 。在阿甘特图上描绘出代表这些根的点 和 ,其中 代表辐角最小的根,且 是按逆时针方向构成的多边形。求 的面积。 在阿甘特图上清晰地描绘点 的轨迹,其中 代表满足 且 的复数 。已知 且 ,求 的取值范围。
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完整解法
8(a)
1. Rearrange the given equation to make $z^5$ the subject and express the right-hand side in exponential form.
To express in exponential form :
The modulus is , and the argument is .
2. Find the general expression for the roots by taking the 5th root.
3. Substitute integer values for $k$ to find the 5 roots within the principal argument range $-\pi < \theta \le \pi$.
Testing consecutive integers for :
For ,
For ,
For ,
For ,
For ,
The 5 roots are .
4. Sketch the roots on an Argand diagram, identifying points $P_1$ to $P_5$.
The roots represent 5 equally spaced points on a circle of radius 2 centered at the origin.
Ordering the roots by smallest argument to largest:
represents
represents
represents
represents (on the positive imaginary axis)
represents
(Self-drawn Argand diagram sketch showing a regular pentagon inscribed in a circle of radius 2, with vertices labelled to in an anticlockwise sequence.)
5. Calculate the area of the polygon $P_1 P_2 P_3 P_4 P_5$.
The polygon is a regular pentagon, which can be divided into 5 identical isosceles triangles extending from the origin.
For each triangle, the two equal sides have length , and the included angle is .
Roots: , , , , .
Area (or )
8(b)
1. Identify the Cartesian equations for the two individual locus conditions.
Let .
The condition represents the region inside and on the circle with centre and radius . Its Cartesian inequality is:
The condition represents a half-line starting from with gradient . Its Cartesian equation is:
2. Find the intersection of the two loci to determine the exact endpoints of the locus of $P$.
Substitute the line equation into the circle's inequality:
This gives .
When , .
When , .
The locus of is the solid line segment connecting the points and .
3. Describe the sketch of the locus of $P$ on the Argand diagram.
On an Argand diagram, indicate the circle centre radius and the half-line from with dashed lines to show the boundaries.
The locus of is sketched by drawing a solid straight line segment from the coordinate to the coordinate .
4. Determine the argument of $z-3-2i$ at the endpoints of the line segment locus.
Let . We need the range of for on the line segment.
At the lower endpoint :
At the upper endpoint :
5. Analyse the continuous variation of the argument to determine the final range.
The segment in the -plane connects to . Because the -coordinates transition from negative () to positive () whilst the -coordinates remain entirely negative, the segment crosses the negative real axis (where ).
At the point of intersection with the negative real axis, the principal argument is .
Moving from to , the argument takes all values in below the real axis, jumps to , and takes all values in above the real axis.
Therefore, the range of is:
The locus of is the line segment from to .
Range of : (or to 3 s.f.)
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