# Thread: NEW USER NEED SUM HELP with this calculus prob. Derivative Prob.

1. ## NEW USER NEED SUM HELP with this calculus prob. Derivative Prob.

hi im jhay from.. phl. civil engr. student .. first time to take calculus..
can u explain to me step by step of finding the derivative of a function x√(x-1) using four step rule .. big tnx in advance..

2. ## Re: NEW USER NEED SUM HELP with this calculus prob. Derivative Prob.

I don't know what you mean by the "four step rule". It is a product of two functions, so use the product rule. The second function also needs the use of the chain rule.

3. ## Re: NEW USER NEED SUM HELP with this calculus prob. Derivative Prob.

I think these are the four steps that iamjhay is talking about:

1. Write $f(x+h)$ and $f(x)$

2. Compute $f(x+h) - f(x)$

3. Simplify $\frac{f(x+h)-f(x)}{h}$

4. Find $\lim_{h\to0}\frac{f(x+h)-f(x)}{h}$

We would call it computing the derivative from the definition.

To iamjhay: I think this identity will get you over the difficult part:

$\sqrt{a}-\sqrt{b}=\frac{a-b}{\sqrt{a}+\sqrt{b}}$

If you're still having trouble, show us the work you've done so far and we'll help you with the next step.

- Hollywood