Stage: 5 Challenge Level: 
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$P$ is a point inside a square $ABCD$ such that $PA= 1$, $PB = 2$ and $PC = 3$. How big is ${\angle APB} $ ? |
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Mathematical reasoning & proof. Pythagorean triples. Sine rule. Cosine rule. Pythagoras' theorem. Circles. Angles. Perpendicular lines. Similarity. Long problems.
Published March 2002.
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email: nrich@damtp.cam.ac.uk
