Is it the notation I've used that is confusing?
f(x) = (1 + x)1/2 is the square root of (1+x), so putting x = 0, f(x)at x = 0 = 1
df(x)/dx = (1/2)(1 + x)-1/2 so putting x = 0, df(x)/dxat x=0 = 1/2
d( df(x)/dx )/dx = -(1/4)(1 + x)-3/2 so putting x = 0, d( df(x)/dx )/dxat x = 0 = -1/4
and so on.
Putting these back into the Maclaurin series f(x) = 1 + x/2 - x2/8 ...
Notice that f(x) is not exactly 1 + x/2. This is just a first-order approximation.