dR/dh = 0,

where R is 2 (hH-h^{2 })^{1/2}

d [2 (hH-h^{2 })^{1/2}] /dh = 0

2 x 1/2 (hH-h^{2})^{-1/2}x (H-2h) = 0

(H-2h)/rt (hH-h) = 0

H-2h=0

h = H/2

this is a part of a physics question and any help is appreciated.

I get it till the 2 x1/2 part, but how did they get the (h-2h) part?

If someone could explain this to me, I'd be super-grateful!