a
(a)
In given condition, if $r$ is radius of curved path travelled by changed particle.
Then,
$\sin \theta \approx \tan \theta=\frac{a}{r}$
or $\quad r=a \operatorname{cosec} \theta \approx a \cot \theta \quad \dots(i)$
Also, $\quad r=\frac{m v}{B q} \quad \dots(ii)$
Combining Eqs. $(i)$ and $(ii)$, we get
$v=\frac{q B}{m} a \cot \theta$
