(a) $\cosh^2 ( -9 ) - \sinh^2 ( -9 ) =$

(b) $\cosh^2 ( -2 ) - \sinh^2 ( -2 ) =$

(c) $\cosh^2 ( 1 ) - \sinh^2 ( 1 ) =$

(d) $\sinh^2 ( 9 ) - \cosh^2 ( 9 ) =$

Note: $\sinh^2 (x)$ is defined as $( \sinh (x) ) ^2$ and $\cosh^2 (x)$ is defined as $( \cosh (x) ) ^2$