Your function as written is monotone decreasing in the variable ah25, and asymptotically approaches -1.25e6 as ah25 gets large. I am not at all sure that this is what you intended your function to do, but that is what it does.
If, instead, you want to find a zero of your function, use fzero instead of fminsearch, because fminsearch looks for a minimum of your function, which, as I just told you, is approximately -1.25e6 for all large enough values of ah25.
Of course, your function is simple enough that you don't need to use fzero to find a root. Inspection shows that ah25 = 1 is the unique solution.
Alan Weiss
MATLAB mathematical toolbox documentation
