Show how identities (6) and (8) in Sec. 35 follow from identities (9) and (6), respectively, in

Question:

Show how identities (6) and (8) in Sec. 35 follow from identities (9) and (6), respectively, in Sec. 34.
(6) cosh2 z − sinh2 z = 1,
(8) cosh(z1 + z2) = cosh z1 cosh z2 + sinh z1 sinh z2
(6) cosh2 z − sinh2 z = 1,
(9) sinh z = sinh x cos y + i cosh x sin y,