# Thread:

1. ## Calculator help

I am trying to graph r^2=2cos(2theta)
in my answer key it's supposed to be an infinity symbol but I get a 4 leaf lemniscate. I am in polar mode so what can be the error?

2. Originally Posted by aaronb
I am trying to graph r^2=2cos(2theta)
in my answer key it's supposed to be an infinity symbol but I get a 4 leaf lemniscate. I am in polar mode so what can be the error?
If $\theta$ lies between $\pi/4$ and $3\pi/4$ (or between $-3\pi/4$ and $-\pi/4$) then $\cos(2\theta)$ is negative and will not have a square root. So the top and bottom leaves of the lemniscate ought not to be there. Maybe you should rewrite the equation as $r = \sqrt{2\cos(2\theta)}$ and see what the calculator makes of that version.
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