

Calculating the area of an annulus from a tangent - jgrahamc
http://blog.jgc.org/2006/09/bouts-best-of-usethesource.html

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twiceaday
I wrote down the equations for the two circles and the line then intersected
the line with the outer circle to get a relationship between the length of the
line and the two radii, calling this equation one. The second equation is the
equation for the area of the annuli given their radii. Solving both equations
for R^2-r^2 and equating gives the answer.

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2muchcoffeeman
What bit of knowledge do you need to understand the 'logical' solution must be
true?

