<p>
Area of a circle.
<p>
A = &pi;r<sup>2</sup>
<p>
Gain of a triode amplifier.
<p>
A = &mu;r<sub>p</sub>R<sub>b</sub>R<sub>cf</sub> / (R<sub>b</sub> + R<sub>cf</sub>)
<p>
Value of a resistor.
<p>
R = 470 k&Omega;
<p>
Reactance of a capacitor.
<p>
X<sub>C</sub> = 1 / (&omega;C).  Where &omega; = 2&pi;f
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