Stage: 5 Challenge Level: 
If for a triangle $ABC$
$\tan(A - B) + \tan(B - C) +
\tan(C - A) = 0$
what can you say about the triangle?
Number theory. Periodicity. epsilons. Generalising. Stage 5 - Reviewed 2012. Manipulating algebraic expressions/formulae. Tangent. Inequality/inequalities. Trigonometric identities. Mathematical reasoning & proof.
Published April 2000.
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