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Premium

Video: Pop your bubble

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Intermediate

Balancing act II

Two squares share a vertex. What’s the angle α?

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Premium

Video: Wheel of fortune II

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Intermediate

Mind the steps

Three similar rectangles and a line segment. Express red in terms of blue and green.

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Intermediate

In and out

An obtuse triangle, of which α is the obtuse angle, with its incircle, two cevians and two tangents. Find a relation between α and β.

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Video: The Bedouin tent

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Beginner

Angle versus height

What is the ratio orange/green?

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Video: Four six four

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Advanced

Alien eye

Two quarter-circles fit inside a square, and five circles fit inside the overlapped region, centred. What is the ratio of the (straight line) segment lengths red to blue? (The segments connect to points of tangency.)

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Intermediate

Pop you bubble

A square with a semicircle and a circle of equal radius. Their tangency point is shown. Prove that the red triangle is equilateral.