Mathf.Pow alternative function?

Hello there!
I have a problem with the Mathf.Pow-Function of the UnityEngine, cause it just allows float values and no double values. But for my program where I can buy Items i need functions like this:
f(x):= 100+1.25^x
And would be the best if x can be a double value in this function aswell.
Is there any way to get something like this? I just wonder that the Unity-Math-Function cant work with double values like the usual c# do.

Would be happy for any solutions and ty for reading my bad english.