An algebra problem by Snehashis Mukherjee

Algebra Level 4

If the point z z in the complex plane describes a circle of radius 2 with centre at the origin, then the point z + 1 z z+\dfrac1z describes a __________ \text{\_\_\_\_\_\_\_\_\_\_} .

hyperbola circle ellipse parabola

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1 solution

Tom Engelsman
Dec 16, 2018

The circular point z z is described by z = 2 e i θ . z = 2e^{i\theta}. If we are interested in the curve described by z + 1 z z + \frac{1}{z} , then:

z + 1 z = 2 e i θ + 1 2 e i θ = 2 ( c o s ( θ ) + i s i n ( θ ) ) + 1 2 ( c o s ( θ ) i s i n ( θ ) ) = 5 2 c o s ( θ ) + i 3 2 s i n ( θ ) z + \frac{1}{z} = 2e^{i\theta} + \frac{1}{2} e^{-i\theta} = 2(cos(\theta) + i \cdot sin(\theta)) + \frac{1}{2}(cos(\theta) - i \cdot sin(\theta)) = \boxed{\frac{5}{2} cos(\theta) + i \cdot \frac{3}{2} sin(\theta)}

which is an ellipse.

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