Given that y = sinh x + k cosh x, show that the least value of y is √(k^2 - 1) and that this occurs at x = ½ ln (k-1)/(k+1) where k is a constant and |k| > 1.

What is mean by the least value?

My solution:

[Substitute x = ½ ln (k-1)/(k+1) ]

By simplifying this I got

Is that a right way? Now what to do?