Level 20

Author:Colin Beveridge

Level 20 of http://euclidthegame.org/Level20.html asks you to construct the outer tangent to two given circles (here, a, with centre A; and b, with centre B). Here's how I'd do it.

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Identify Opposite, Adjacent, Hypotenuse
Folding door
Right Triangle Trig Ratios: sin, cos, tan
Quadratic Equations: the Geometry of the Relationships Between the Coefficients and the Solutions

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Slide 7
Gear train
Secant to Tangent Line
Circumcenter
Sequential limits: divergent

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Arithmetic Mean
Trigonometric Functions
Geometric Transformations
Pythagoras or Pythagorean Theorem
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