Eqn 16 Help

cosh(z) = ½ (e^z + e^–z) so cosh²(z) = ¼ (e^2z + e^–2z + 2)

sinh(z) = ½ (e^z – e^–z) so sinh²(z) = ¼ (e^2z + e^–2z – 2)

By subtraction, cosh²(z) – sinh²(z) = ¼ (2+2) = 1

Putting z = x/a, cosh²(x/a) – sinh²(x/a) = 1