14.如图所示,在平行六面体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中,NNN为A1C1A_{1}C_{1}A1C1与B1D1B_{1}D_{1}B1D1的交点,MMM为DD1DD_{1}DD1的中点,若AB→=a→\overrightarrow{AB}=\overrightarrow{a}AB=a,AD→=b→\overrightarrow{AD}=\overrightarrow{b}AD=b,AA1→=c→\overrightarrow{AA_1}=\overrightarrow{c}AA1=c,则MN→=(\overrightarrow{MN}=(MN=( )))