Graphing Calculator Drawing: Rotation

by Justin Skycak (x.com/justinskycak) on May 10, 2019

Absolute value graphs can be rotated to draw stars.

This post is part of the book Graphing Calculator Drawing Exercises. Suggested citation: Skycak, J. (2019). Graphing Calculator Drawing: Rotation. In Graphing Calculator Drawing Exercises. https://justinmath.com/graphing-calculator-drawing-rotation/

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Setup. Navigate to https://www.desmos.com/calculator. Be sure to sign in so that you can save your graph.

Demonstration - Rotation. Observe the graph as you type each of the following inputs. In general, a graph can be rotated by an angle of $\theta$ about the origin by replacing $x$ and $y$ with the following expressions:

$\begin{align*} x &\rightarrow x\cos\theta + y\sin\theta \\[5pt] y &\rightarrow y\cos\theta - x\sin\theta \end{align*}$

Note that $\theta$ should be given in radians, and one can convert degrees to radians by multiplying by the conversion factor $\frac{\pi}{180}.$

$\begin{align*} &y\cos \frac{\pi}{6} - x \sin \frac{\pi}{6} = \left( x \cos \frac{\pi}{6} + y\sin\frac{\pi}{6} \right)^2 \\[10pt] &\left( \frac{x \cos \frac{\pi}{4} + y \sin \frac{\pi}{4} }{4} \right)^{2}+\left( \frac{ y \cos \frac{\pi}{4} - x \sin \frac{\pi}{4} }{2} \right)^{2}=1 \end{align*}$

Exercise. Reproduce the graph below by drawing an absolute value function and then rotating it a fifth of a circle counterclockwise.