b
\(\begin{aligned} \Delta \ell=\sqrt{\ell^2{ }_2+\ell^2{ }_1-2 \ell_1 \ell_2 \cos \theta} \\ =\sqrt{\ell^2+\ell^2-2 \ell^2 \cos \theta}=\ell \sqrt{2(1-\cos \theta)}=\end{aligned}\)
\(= \,\ell \sqrt {2\,\, \times \,\,2\,{{\sin }^2}\frac{\theta }{2}} \,\, = \,\,2\ell \sin \frac{\theta }{2}\)
