Prove that \[(\overrightarrow{a}+\overrightarrow{b}).(\overrightarrow{a}-\overrightarrow{b})={{\left| \overrightarrow{a} \right|}^{2}}+{{\left| \overrightarrow{b} \right|}^{2}}\], if and only if \[\overrightarrow{a}\] , \[\overrightarrow{b}\] are perpendicular, given\[\overrightarrow{a}\ne 0,\overrightarrow{b}\ne 0\].