15.如图,在平行六面体ABCD−A1B1C1D1ABCD-A_{1}B_{1}C_{1}D_{1}ABCD−A1B1C1D1中AB→=a→\overrightarrow{AB}=\overrightarrow{a}AB=a,AD→=b→\overrightarrow{AD}=\overrightarrow{b}AD=b,AA1→=c→\overrightarrow{A{A_1}}=\overrightarrow{c}AA1=c,OOO为D1BD_{1}BD1B的中点,则用向量a→\overrightarrow{a}a,b→\overrightarrow{b}b,c→\overrightarrow{c}c可表示向量DO→\overrightarrow{DO}DO为((( )))