I believe the equation is
[tex]1=(1-\sin^2\theta)(1+\tan^2\theta)[/tex]
which is true for all [tex]\theta[/tex], so I guess you're supposed to show why (i.e. establish the identity). Recall that
[tex]\cos^2\theta+\sin^2\theta=1[/tex]
from which we can derive
[tex]\cos^2\theta=1-\sin^2\theta[/tex]
[tex]1+\tan^2\theta=\sec^2\theta[/tex]
So the original equation is equivalent to
[tex]1=\cos^2\theta\sec^2\theta[/tex]
and since [tex]\sec\theta=\dfrac1{\cos\theta}[/tex], the right side reduces to 1.
