In \(\triangle OPR\)
\(\vec{a}=\vec{c}+\overrightarrow{R P}\)
and \(\operatorname{In} \triangle ORQ\)
\(\overrightarrow{ b }=\overrightarrow{ c }+\overrightarrow{ RQ }\)
Step \(2:\) Equation solving
Adding \(eq ^{ n }(1)\) and (2)
\(\vec{a}+\vec{b}=2 \vec{c}+\overrightarrow{R P}+\overrightarrow{R Q}\)
Since \(R\) is midpoint of \(PQ\), therefore \(\overrightarrow{ RP }=-\overrightarrow{ RQ }\) \(\Rightarrow \vec{a}+\vec{b}=2 \vec{c}+\overrightarrow{R P}-\overrightarrow{R P}\)
\(\Rightarrow \vec{a}+\vec{b}=2 \vec{c}\)