Answer
$${\text{co}}{{\text{s}}^2}t - {\sin ^2}t = 1$$
Work Step by Step
$$\eqalign{
& {\text{The fundamental identity for hyperbolic functions is:}} \cr
& {\text{ co}}{{\text{s}}^2}t - {\sin ^2}t = 1 \cr
& {\text{See the page 492 in this section}}{\text{.}} \cr} $$
