Hyperbolic functions

59 cosh  x = 1 + x 2 2 ! + x 4 4 ! + .. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429294402/968186b9-ffa4-4f2b-aa54-7fc58a604561/content/eq811.tif"/> (which is valid for all values of x)