[Math] Why is wolfram alpha plotting this differently

3dgraphing-functionswolfram alpha

I have an equation for a cylinder as $x^2+(y-b)^2=a^2$ for some $a$ and $b$.
so I just plugged in $b=2$ and $a=1$ and tried to plot it using wolfram alpha, and the 3D plot looked like half a cylinder, like this.

Why am I not getting a 3D plot for a cylinder instead?

Best Answer

The only way I've got for making WolframAlpha recognise this as a request for a 3D plot in which one variable happens to be absent, is:

plot [x^2]+[(y-2)^2] + 0.000000001z = 1

This will not quite produce the right plot, but it's very close; I've had to add a term which is nearly zero, to make WA understand the $z$ term.

I have reported as a bug the fact that ContourPlot3D[x^2+(y-2)^2==1, {x, -1, 1}, {y, 1, 3}, {z, -4, 4}] is not recognised correctly.

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