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2. Nelsen, R.B.: Proofs without Words II: more Exercises in Visual Thinking (Classroom Resource Material), by Roger B. Nelsen (2001)
3. Alsina, C., Nelsen, R.B.: [C] Math Made Visual: Creating Images For Understanding Mathematics. The Mathematical Association of America, New York (2006)
4. Cut-The-Knot: Geometry articles, theorems, problems. http://www.cut-the-knot.org/geometry.shtml (2009)
5. Cut-The-Knot: Pythagorean theorem. http://www.cut-the-knot.org/pythagoras/index.shtml (2009)