Answer
$\Rightarrow \sin \theta =\dfrac {\sqrt {7}}{4}$
Work Step by Step
$\sin \theta =\pm \sqrt {1-\cos ^{2}\theta }=\pm \sqrt {1-\left( \dfrac {3}{4}\right) ^{2}}=\pm \dfrac {\sqrt {7}}{4}$
in quadrant 1 sine is positive so
$\Rightarrow \sin \theta =\dfrac {\sqrt {7}}{4}$
