1. May 18, 2013

### Dr_Kesha_PHD

Okay so I'm in Calculus 1 and we are working on derivatives. I understand it all but I have been having some trouble with some basic math skills that I cannot remember from high school and I can't seem to find a good tutorial anywhere online.

I am having problems with multiplying fractional exponents and also adding them. I know that when you have multiply or divide exponents it is always like x^6*x^2=x^6+2=x^8.

But it is throwing me off when the exponents have a fraction or radical because I don't remember these basic skills from high school.

Let me give you an example of a problem that I am struggling with...

Differentiate:
H(u)=(u-sqrt(u))(u+sqrt(u))

I know that this means to find the derivative of this equation. So I use the product rule which is the derivative of the 1st term multiplied by the second term, plus the derivative of the second term multiplied by the first time.
This gives me:

(u-sqrt(u))(1+1/2u^-1/2)+(u+sqrt(u))(1-1/2u^-1/2)

How do I perform -sqrt(u) multiplied by 1/2u^-1/2 ?

Can someone show me where I am going wrong because I am coming up with -1/2.

2. May 18, 2013

### micromass

Staff Emeritus
That is correct:

$$-\frac{1}{2}\sqrt{u} u^{-1/2} = -\frac{1}{2} u^{1/2} u^{-1/2} = -\frac{1}{2} u^{1/2 - 1/2} = -\frac{1}{2}u^0 = -\frac{1}{2}$$

3. May 18, 2013

### Dr_Kesha_PHD

Well then I'm not totally sure where I am going wrong because I keep coming up with the wrong answer to the problem. But thank you very much for showing me that that wasn't where I am making my mistake.

4. May 18, 2013

### HallsofIvy

Staff Emeritus
Rather than use the "product rule", I would just go ahead and multiply:
$H(u)= u^2- u$

So H'(u)= 2u- 1

I know that this means to find the derivative of this equation. So I use the product rule which is the derivative of the 1st term multiplied by the second term, plus the derivative of the second term multiplied by the first time.
This gives me:

(u-sqrt(u))(1+1/2u^-1/2)+(u+sqrt(u))(1-1/2u^-1/2)

How do I perform -sqrt(u) multiplied by 1/2u^-1/2 ?

Can someone show me where I am going wrong because I am coming up with -1/2.