OABC OABC\,OABC is a parallelogram with OA→=a\overrightarrow{OA}=\mathbf aOA=a and OC→=c\overrightarrow{OC}=\mathbf cOC=c, where A A\,A and C C\,C are adjacent to OOO. Express OB→\overrightarrow{OB}OB and AC→\overrightarrow{AC}AC in terms of a \mathbf a\,a and c\mathbf cc.