\(|\overrightarrow {OB} |\, = a\)
\(\overrightarrow {OB} - \overrightarrow {OA} = \overrightarrow {AB} = \Delta \overrightarrow {a\,} \) \( \Rightarrow \,\,\,|\Delta \overrightarrow {a\,} |\, = AB\)
\(AB = a . d\theta \)
\(|\Delta \overrightarrow {a\,} |\, = \,a\,d\theta \)
\(|\overrightarrow {OB} | - |\overrightarrow {OA} |\) \( \Rightarrow \,\,\,a - a = 0\)
\(\Delta a = 0\)