$\begin{array}{l} M{P^2} = MQ.MN\left( {định lý 1} \right)\\ \Rightarrow 36 = 3,6.MN\\ \Rightarrow MN = 10\left( {cm} \right)\\ QN = MN - MQ = 10 - 3,6 = 6,4\left( {cm} \right)\\ P{Q^2} = MQ.QN = 3,6.6,4 = \dfrac{{576}}{{25}}\left( {cm} \right)(định lý 2)\\ \Rightarrow PQ = 4,8\left( {cm} \right) \end{array}$