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Feb/Mar 2016 p32 q10
1977
(a) Find the complex number z satisfying the equation \(z^* + 1 = 2iz\), where \(z^*\) denotes the complex conjugate of \(z\). Give your answer in the form \(x + iy\), where \(x\) and \(y\) are real.
(b) (i) On a sketch of an Argand diagram, shade the region whose points represent complex numbers satisfying the inequalities \(|z + 1 - 3i| \leq 1\) and \(\text{Im } z \geq 3\), where \(\text{Im } z\) denotes the imaginary part of \(z\).
(ii) Determine the difference between the greatest and least values of \(\arg z\) for points lying in this region.
Solution
(a) Let \(z = x + iy\). Then \(z^* = x - iy\). Substitute into the equation:
\(x - iy + 1 = 2i(x + iy)\)
Equate real and imaginary parts:
Real: \(x + 1 = -2y\)
Imaginary: \(-y = 2x\)
From \(-y = 2x\), we have \(y = -2x\).
Substitute \(y = -2x\) into \(x + 1 = -2y\):
\(x + 1 = 4x\)
\(3x = 1\)
\(x = \frac{1}{3}\)
Then \(y = -2x = -\frac{2}{3}\).
Thus, \(z = \frac{1}{3} - \frac{2}{3}i\).
(b)(i) The region is a circle with center \(-1 + 3i\) and radius 1, intersecting the line \(\text{Im } z = 3\). Shade the region inside the circle and above the line.
(b)(ii) The greatest and least values of \(\arg z\) are determined by the intersection points of the circle and the line. The difference in \(\arg z\) is 0.588 radians (accept 33.7°).