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Nov 2017 p32 q7
1965
The complex number \(1 - (\sqrt{3})i\) is denoted by \(u\).
(i) Find the modulus and argument of \(u\).
(ii) Show that \(u^3 + 8 = 0\).
(iii) On a sketch of an Argand diagram, shade the region whose points represent complex numbers \(z\) satisfying both the inequalities \(|z - u| \leq 2\) and \(\text{Re } z \geq 2\), where \(\text{Re } z\) denotes the real part of \(z\).
Solution
(i) The modulus of \(u = 1 - (\sqrt{3})i\) is given by \(\sqrt{1^2 + (-\sqrt{3})^2} = \sqrt{1 + 3} = 2\).
The argument is \(\arctan\left(\frac{-\sqrt{3}}{1}\right) = -\frac{\pi}{3}\) or \(-60^\circ\).
(ii) To show \(u^3 + 8 = 0\), calculate \(u^3 = (1 - (\sqrt{3})i)^3\).
Using the binomial theorem or De Moivre's theorem, \(u^3 = 2^3 \left(\cos(-\pi) + i\sin(-\pi)\right) = -8\).
Thus, \(u^3 + 8 = -8 + 8 = 0\).
(iii) On the Argand diagram, draw a circle centered at \(1 - (\sqrt{3})i\) with radius 2.
Draw the vertical line \(\text{Re } z = 2\).
Shade the region to the right of the line and within the circle.