Let \(M\) be the intersection of the diagonals \(AC\) and \(BD\) of cyclic quadrilateral \(ABCD\). If \(|AB|=5\), \(|CD|=3\), and \(m(\widehat{AMB}) = 60^\circ\), find the circumradius of the quadrilateral. If the circumradius is expressed in the form \(\frac{a\sqrt{b}}{c}\), where \(a\), \(b\), and \(c\) are integers, what is the value of \(a + b + c\)?

Let's think step by step and output the final answer within \boxed{}.
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