#### Answer

$\displaystyle \sin\frac{3\theta}{10}$

#### Work Step by Step

Comparing with the Half-Angle Identity,
$\displaystyle \sin\frac{A}{2}=\pm\sqrt{\frac{1-\cos A}{2}}$
if we replace $A$ with $\displaystyle \frac{3\theta}{5}$ in the identity,
the RHS equals the given expression.
So, the LHS $=\displaystyle \sin\frac{A}{2}$= $\displaystyle \sin\frac{\frac{3\theta}{5}}{2}=\sin\frac{3\theta}{10}$