Answer
$\dfrac{\sqrt {2-\sqrt 2}}{2}$
Work Step by Step
Write the correct identity such as: $\sin (a/2)=\sqrt {\dfrac{1-\cos a}{2}}$
Plug the data into the above equation.
$\sqrt {\dfrac{1-\cos (45)}{2}}=\sqrt {\dfrac{1-(\sqrt 2/2)}{2}}$
or, $=\sqrt {\dfrac{2-\sqrt 2}{4}}$
or, $=\dfrac{\sqrt {2-\sqrt 2}}{2}$
