how can i determine wether a circle can be circumscribed about a quadrilateral?
rules for circle circumscribing
3
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circles
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2Look at opposite angles. A convex quadrilateral is cyclic if and only if its opposite angles add up to $\pi$ ($= 180^\circ$). – 2011-03-14
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0See http://en.wikipedia.org/wiki/Cyclic_quadrilateral – 2011-03-14