Start by explaining what the British flag theorem is. Be sure to include at least one example. Then show students its proof. This page will surely help you both explain the theorem and its proof. The proof involves the Pythagorean theorem and adding equations. Using the theorem requires solving quadratic equations by taking square roots. Microsoft gave the interview question (see below), which is easy to solve using the British flag theorem. Have your students solve it to make sure they're able to apply the theorem.
\(ABCD\) is a rectangle. Point \(P\) is 11 units from \(A,\) 13 units from \(B,\) and 7 units from \(C.\) What is the distance from \(P\) to \(D?\) Note: the point \(P\) could be inside or outside the rectangle. The solution can be found here.
TODO: Find an example to help explain the theorem.