扩展：“两个直角”意味着圆

Given that there are 2 opposite right angles in a quadrilateral, the quadrilateral is cyclic. This is so because opposite angles sum up to $180^\circ$. Here's an example diagram:

In fact, the right angles do not have to be opposite angles! As long as they intercept the same side, the quadrilateral is cyclic. For example, take a quadrilateral $ABCD$ with $\angle CAD = \angle CBD = 90^\circ$. Just look below!

In both of these diagrams, $CD$ (which the diagonal intercepts) is the diameter of the circumcircle.