(a)
Given, $(\overrightarrow{\mathbf{a}} \times \overrightarrow{\mathbf{b}})^{2} + (\overrightarrow{\mathbf{a}} \bullet \overrightarrow{\mathbf{b}})^{2} = {\overrightarrow{\mathbf{a}}}^{2}{\overrightarrow{\mathbf{b}}}^{2}\sin^{2}\theta + {\overrightarrow{\mathbf{a}}}_{2}{\overrightarrow{\mathbf{b}}}_{2}\cos^{2}\theta = {\overrightarrow{\mathbf{a}}}^{2}{\overrightarrow{\mathbf{b}}}^{2}$