QUOTE(xClairex @ Jan 15 2007, 09:37 PM)

aah so dont get all this basic stuff

**the question is**

find f'(4) where f(x)= x-1/ ( root x)

so far i have f(x)= (x to the power of 1/2) - (x to the power of -1/2)

this is worth 5 marks so i think i would then do

f'(x) = (x to the power of -1/2) - (x to the power of -3/2)

then sub in 4

f'(4) = (4 to the pwer of -1/2) - (4 to the power of -3/2)

but this is a non calculator question so i dont know what to do next

can anyone help?

You are right in what you have done so far. You are making things difficult for yourself by trying to calculate the f'(4) using the form of the derivative you have already. Rewrite the derivative as: 1/x^1/2 - 1/x^3/2

This will give 1/2 - 1/8 as (4^3)^1/2 is (64)^1/2.

Therefore your final answer should be 3/8. All i did was use the law of indices to write the negative powers x^-n as 1/x^n

This makes the evaluation part a lot easier.

Hope that helped.