Je te propose deux méthodes
f(x) = 1/[2 sqrt(x)] = x^(-1/2) / 2
f'(x) = -1/2 x^(-1/2-1) / 2 = -1/2 x(-3/2) / 2 = -1 / [ 4x sqrt(x) ]
on pose
u(x) = 1
u'(x) = 0
v(x) = 2 sqrt(x)
v'(x) = 1 / sqrt(x)
f'(x) = [ -1/sqrt(x) ] / [ 2 sqrt(x) ]² = -1 / [ 4x sqrt(x) ]