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If the following vectors are coplanar find the value of a.A⃗=i^+j^−ak^B⃗=2i^−j^+3k^C⃗=−i^+2j^−k^ \begin{array}{l} \vec{A}=\hat{i}+\hat{j}-a \hat{k} \ \vec{B}=2 \hat{i}-\hat{j}+3 \hat{k} \ \vec{C}=-\hat{i}+2 \hat{j}-\hat{k} \end{array}