@John May Oh ok, thank you!

Thanks a LOT! This is exactly what I wanted! This is just perfect. Thanks again!

@tomleslie 

Sure:
a = 4.870
b = 0.429
f = 2000*10^6
c = 3*10^8
nNLOS=21
A = -20.9

What do you mean by creating a worksheet? how can we do that in maple?

@tomleslie 

Thanks for your detailed response.

As far as the simple errors are concerned, I have corrected them. I was actually transitioning from MATLAB to Maple and hence these errors.

1. R and h are not really independent. In my system model, these are two adjacent sides of a right angle triangle. But the third is also unknown and the two angles(other than right angle) are also unknown. 

2. Yes, I want partial derivative of R w.r.t h to be 0.

3. Ideally, yes, I want second derivative to be negative too, but that doesn't really matter. I know it's negative because I plotted R for a range of h in MATLAB and the curve has a concave down shape, like an upside-down letter 'U'.