c
\(\,\mathop A\limits^ \to  \,\, + \,\,\mathop B\limits^ \to  \,\, = \,\,\mathop {{R_1}}\limits^ \to  \)

અને  \(\,\mathop A\limits^ \to  \,\, - \,\,\mathop B\limits^ \to  \,\, = \,\,\mathop {{R_2}}\limits^ \to  \)

\( \Rightarrow \,\,R_1^2\,\, + \;\,R_2^2 = \,\,\left( {{A^2}\,\, + \;\,{B^2}\,\, + \;2AB\cos \theta } \right)\,\, + \;\,\,\left( {{A^2}\,\, + \;\,{B^2}\,\, - \,\,2AB\,\cos \theta } \right) = \,\,2\left( {{A^2}\,\, + \;\,{B^2}} \right)\;\)