https://www.explainxkcd.com/wiki/api.php?action=feedcontributions&feedformat=atom&user=217.149.210.16explain xkcd - User contributions [en]2026-07-06T13:35:39ZUser contributionsMediaWiki 1.30.0https://www.explainxkcd.com/wiki/index.php?title=1184:_Circumference_Formula&diff=302211184: Circumference Formula2013-03-11T06:33:48Z<p>217.149.210.16: Earth Prime</p>
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<div>{{comic<br />
| number = 1184<br />
| date = March 10, 2013<br />
| title = Circumference Formula<br />
| image = circumference_formula.png<br />
| titletext = Assume r' refers to the radius of Earth Prime, and r'<nowiki/>' means radius in inches.<br />
}}<br />
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==Explanation==<br />
The circumference of a circle is C = 2πr, where ''r'' is the radius of the circle. Randall then makes a footnote about ''r'', but he uses <sup>2</sup>. This creates a typographical ambiguity, since the superscript 2 can also be an exponent (as in ''x''<sup>2</sup>). Randall's formula now looks like a strange hybrid of the correct formula and A = πr<sup>2</sup>, which is the formula for the ''area'' of the circle. The term ''Earth Prime'' refers to the television series Sliders.<br />
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In the title text, r' normally means the derivative of r, and r'<nowiki/>' the double derivative of r, whereas ' can be read as "prime", while '<nowiki/>' can refer to inches.<br />
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==Transcript==<br />
:Circumference of a circle:<br />
:2πr<sup>2</sup><br />
:<sup>2</sup>The circle's radius<br />
<br />
{{comic discussion}}<br />
[[Category:Math]]</div>217.149.210.16