All of Maximas and Minimas!

Algebra Level 4

Find the sum of maximum limiting value and minimum value of z i \displaystyle\large \left| z^i \right| where z C \displaystyle\large z\in \mathbb{C} and the argument of z z lies in the principal range.


The answer is 23.18390655.

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1 solution

Kishore S. Shenoy
Sep 19, 2015

Let z = r e i θ \displaystyle z = re^{i\theta}

e \left|e\right|

z i = r i e θ = r i e θ = e ln r i e θ = e θ [ e ln r i = e i ln r = 1 ] \large\displaystyle \begin{aligned} \left| z^i \right| &=\left| r^i \right| \left| e^{-\theta} \right|\\ &=\left| r^i \right| e^{-\theta} \\&=\left| e^{\ln r^i} \right| e^{-\theta}\\&=e^{-\theta} &\left[\because \left|e^{\ln r^i}\right| = \left|e^{i\ln r}\right| = 1\right] \end{aligned}

Now taking Principle Angles,

π < θ π π > θ p i e π > e θ e π \begin{aligned}-\pi&<\theta\le\pi\\\Rightarrow \pi &> -\theta\ge -pi\\\therefore e^{\pi}&>e^{\theta}\ge e^{-\pi}\end{aligned}

max + min = e π + e π = 23.18390655 \therefore \max + \min = e^{\pi} + e^{-\pi} = \boxed{23.18390655}

Moderator note:

Good approach.

You use the equation ln ( r i ) = i ln ( r ) \ln(r^i )=i\ln(r) ... does that hold for all positive r r ?

Otto Bretscher - 5 years, 8 months ago

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