Basic concepts of vector - class-XI

If $\overrightarrow A ,\overrightarrow B $ and $\overrightarrow C $ are vectors such that $\left| {\overrightarrow B } \right| = \left| {\overrightarrow C } \right|$ , then  $\left{ {\left( {\overrightarrow A  + \overrightarrow B } \right)} \right. \times \left. {\left( {\overrightarrow A  + \overrightarrow C } \right)} \right} \times \left( {\overrightarrow B  \times \overrightarrow C } \right).\left( {\overrightarrow B  + \overrightarrow C } \right) = 1 $  these relation is ?