First, have students derive the formula for the circumradius of an equilateral triangle, with sides of length \(s.\) The correct formula is \(s / \sqrt{3}.\) After that, have students derive the formula for the circumradius of an isosceles triangle, with base \(b,\) height \(h,\) and slant height \(s.\) The correct formula is \(r = s^2 / (2h).\) Its derivation can be seen here.