I have an issue with a problem.

I have the following function...

$\displaystyle Cost: C^2 = x^2 + 100(x)^[1/2] + 50$

I'm asked to find and interpret the marginal cost dC/dx at x = 5.

Using implicit differentiation, which isn't really too hard because each variable is already on separate sides of the equation, I came up with the answer of

$\displaystyle x + 25(x)^{-1/2} = C$.

However, when I plug 5 into the derivative, I get an answer of about 16.18. I should be getting 0.94. Can someone give me a tip on what I've done wrong here?

Thank you.