Wasn't too sure where to put this one, so feel free to yell at me if it's in the wrong forum. :)

I'm having some difficulty starting this problem,

Show that

$\displaystyle (\cosh x + \sinh x)^k + (\cosh x - \sinh x)^k = 2\cosh kx$

I thought about proof by induction, or expanding the left hand side using the binomial theorem. But both times I got stuck. Any help would be much appreciated. Thanks

Stonehambey