# rules for circle circumscribing

how can i determine wether a circle can be circumscribed about a quadrilateral?

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Look at opposite angles. A convex quadrilateral is cyclic if and only if its opposite angles add up to $\pi$ ($= 180^\circ$). –  t.b. Mar 14 '11 at 17:49
–  lhf Mar 14 '11 at 18:23