AS maths help!!!

I need help with differentiation as I'm quite stuck on a homework.
The question reads:
f(x)=1/x - √x
Find the value of f''(x) when x=4

I have an answer of 1/16, however I'm not sure if this is correct or not!
Any help would be greatly appreciated, thank you!

I think it's correct. f'(x) = -x^-2 -(1/2)x(-1/2)
f''(x) = 2x^(-3) + (1/4)x^(-3/2) = 2/(x^3) + 1/(4x^(3/2))
f''(4) = 2/64 + 1/32 = 4/64 = 1/16.

I assume $f(x) = \dfrac{1}{x} - \sqrt{x}$

http://www.thestudentroom.co.uk/showthread.php?p=50436265

Can you show what you got when you differentiated

f(x)=1/x - √x

Rearranged - f(x)=x^-1-x^1/2

1st Differentiation -

f'(x)=-x^-2-1/2x^-1/2

2nd Differentiation -

f"(x)=2x^-3+1/4x^-3/2

Substitute in the x -

= 2x4^-3 + 1/4x4^-3/2

= 1/32 + 1/32

=1/16

Yes

My eyes are old - I was only looking at f ' (4)

Haha, it is quite small! So 1/16 is the correct answer? Thank you all very much for the help!!