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June 2016 p33 q9
1974
The complex numbers \(-1 + 3i\) and \(2 - i\) are denoted by \(u\) and \(v\) respectively. In an Argand diagram with origin \(O\), the points \(A, B\) and \(C\) represent the numbers \(u, v\) and \(u + v\) respectively.
Sketch this diagram and state fully the geometrical relationship between \(OB\) and \(AC\).
Find, in the form \(x + iy\), where \(x\) and \(y\) are real, the complex number \(\frac{u}{v}\).
Prove that angle \(AOB = \frac{3}{4}\pi\).
Solution
(i) Obtain \(u + v = 1 + 2i\). In an Argand diagram, show points \(A, B, C\) representing \(u, v\) and \(u + v\) respectively. State that \(OB\) and \(AC\) are parallel and \(OB = AC\).
(ii) Multiply numerator and denominator of \(\frac{u}{v}\) by \(2 + i\) to simplify. Simplify the numerator to \(-5 + 5i\) and the denominator to 5. The final answer is \(-1 + i\).
(iii) Find \(\arg\left(\frac{u}{v}\right)\) to determine \(\angle AOB\). Show sufficient working to justify the given answer \(\frac{3}{4}\pi\).