R=2(hH-h^2)^1/2

Let hH-h^2=t

R=2t^1/2

dR/dt=2x(1/2)xt^-(1/2)

=t^-(1/2)

t=hH-h^2

dt/dh=H-2h

dR/dh=(dR/dt)x(dt/dh)= t^-(1/2)x(H-2h)

=(H-2h)/(hH-h^2)^1/2

If we put dR/dh=0

dR/dh=(H-2h)/(hH-h^2)^1/2=0

First thing is that denominator should not be zero hence hH-h^2 is non zero

Therefore h can't be zero and can't be equal to H

numerator should be zero, thus H=2h