Solve the equation , giving your roots in the form , where and . Sketch on an Argand diagram the points and representing these roots, where represents the root with the smallest argument and is a polygon described in an anticlockwise sense. Find the area of . On an Argand diagram sketch clearly the locus of where represents the complex number such that and . Find the range of values of , given that and .
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Worked solution
8(a)
1. Rearrange the equation and express the right-hand side in exponential form.
2. Formulate the general expression for the roots.
3. Find the 5 specific roots in the required principal argument range.
Evaluating for to keep :
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 .
has the smallest argument, so (in the 3rd quadrant).
Following an anticlockwise direction:
(in the 4th quadrant)
(in the 1st quadrant)
(on the positive imaginary axis)
(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 .
Roots: .
Area = or units.
8(b)
1. Identify the two loci and formulate equations to find their intersection.
The inequality represents the region on and inside a circle centred at with radius .
The equation represents a half-line (ray) starting from the point with a direction vector corresponding to . This ray lies on the line passing through with gradient , so its Cartesian equation is for .
To sketch the locus, we must find the intersection of this ray with the boundary of the circle, .
2. Solve for the endpoints of the line segment representing the locus of $P$.
Substitute into the circle's boundary equation:
Solving gives or .
When , .
When , .
Both points satisfy and , so they lie on the half-line.
The locus of is the line segment connecting and . (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 . We need the range of as varies along the segment.
At the endpoint :
At the endpoint :
Since this point is in the third quadrant:
4. Determine the full range of $\arg(z-3-2i)$ by checking the transition of the angle along the segment.
As moves along the segment from to , the vector moves along a straight line from to .
This path crosses the negative real axis when the imaginary part of is zero. This occurs at .
On the line , when , .
At this intercept, , which has an argument of exactly .
Thus, the argument monotonically increases from up to in the second quadrant. Upon crossing the negative real axis, the principal argument wraps to and increases to in the third quadrant.
The range of values is therefore:
or
or
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