How do you simplify 1 – sec² t?

Ans: $- {\tan}^{2} t$
$\left(1 - {\sec}^{2} t\right) = \left(1 - \frac{1}{{\cos}^{2} t}\right) = \frac{{\cos}^{2} t - 1}{\cos} ^ 2 t =$
$= - \frac{{\sin}^{2} t}{{\cos}^{2} t} = - {\tan}^{2} t$