Answer
$1-\cos \ \theta $
Work Step by Step
The half-angle Identity for sine can be expressed as:
$\sin {(\dfrac{\theta}{2})}=\pm\sqrt{\dfrac{1-\cos{\theta}}{2}}$
On squaring both sides of the identity above, we obtain:
$\sin^2 {(\dfrac{\theta}{2})}=\dfrac{1-\cos{\theta}}{2}$
Therefore, the missing equation or expression in the given statement is $1-\cos \ \theta $
