Answer
$1$
Work Step by Step
$\dfrac {\sin ^{2}\theta -\cos ^{2}\theta }{\sin ^{4}\theta -\cos ^{4}\theta }=\dfrac {\sin ^{2}\theta -\cos ^{2}\theta }{\left( \sin ^{2}\theta -\cos ^{2}\theta \right) \left( \sin ^{2}\theta +\cos ^{2}\theta \right) }=\dfrac {1}{\sin ^{2}\theta +\cos ^{2}\theta }=\dfrac {1}{1}=1$
