Ta có: \(\overrightarrow {MN} = \overrightarrow {MC} + \overrightarrow {CN} \)

⇔ \(\overrightarrow {MN} = \overrightarrow {MB} + \overrightarrow {BC} + \overrightarrow {CN} = - \frac{1}{3}\overrightarrow {BC} + \overrightarrow {BC} + \frac{5}{4}\overrightarrow {CA} = \frac{2}{3}\overrightarrow {BC} + \frac{5}{4}\overrightarrow {CA} \)

⇔ \({\overrightarrow {MN} ^2} = {\left( {\frac{2}{3}\overrightarrow {BC} + \frac{5}{4}\overrightarrow {CA} } \right)^2}\)

⇔ \({\overrightarrow {MN} ^2} = {\left( {\frac{2}{3}\overrightarrow {BC} } \right)^2} + \frac{5}{3}\overrightarrow {BC} .\overrightarrow {CA} + {\left( {\frac{5}{4}\overrightarrow {CA} } \right)^2}\)

⇔ \({\overrightarrow {MN} ^2} = {\left( {\frac{2}{3}\overrightarrow {BC} } \right)^2} - \frac{5}{3}\overrightarrow {CB} .\overrightarrow {CA} + {\left( {\frac{5}{4}\overrightarrow {CA} } \right)^2}\)

⇔ \({\overrightarrow {MN} ^2} = \frac{4}{9}{\overrightarrow {BC} ^2} - \frac{5}{3}\left| {\overrightarrow {BC} } \right|.\left| {\overrightarrow {CA} } \right|{\rm{cos}}\left( {\overrightarrow {CB} ,\overrightarrow {CA} } \right) + \frac{{25}}{{16}}{\overrightarrow {CA} ^2}\)

⇔ \({\overrightarrow {MN} ^2} = \frac{4}{9}{\overrightarrow {BC} ^2} - \frac{5}{3}\left| {\overrightarrow {BC} } \right|.\left| {\overrightarrow {CA} } \right|{\rm{cos}}\widehat {CBA} + \frac{{25}}{{16}}{\overrightarrow {CA} ^2}\)

⇔ \({\overrightarrow {MN} ^2} = \frac{4}{9}{a^2} - \frac{5}{3}a.a{\rm{cos60}}^\circ + \frac{{25}}{{16}}{a^2}\)

⇔ \({\overrightarrow {MN} ^2} = \frac{4}{9}{a^2} - \frac{5}{6}{a^2} + \frac{{25}}{{16}}{a^2} = \frac{{169}}{{144}}{a^2}\)

⇔ \(MN = \frac{{13}}{{12}}a\)

Vậy \(MN = \frac{{13}}{{12}}a\).