# Differentiate TU= 4 sqr x1 + x2

1. Oct 4, 2009

### feelgooddan

TU= 4 sqr x1 + x2

differentiate by x1 if x1=9 and x2=8

2. Oct 4, 2009

### Hobse

Re: differentiate

First of all, if you actually plug those numbers in there at this particular moment and then differentiate, you're going to end up with an answer of zero, so you shouldn't do that.

Differentiating anything with a sqrt should go something like this:

Firstly, move the sqrt into an exponent form.

sqrt( x ) = x^(1/2)

Now, take the derivative with respect to x (x1 in your case), using the power rule.

d/dx (x^(1/2)) = (1/2)x^(-1/2) = 1 / [2*(sqrt(x))]

If it applies, which I don't believe it does in your example, don't forget the chain rule.

Hope that helps.

3. Oct 4, 2009

### feelgooddan

Re: differentiate

Thank you! But I guess you forgot 4, is not it? So, the answer gonna be 2 / (sqrt(x1))

I'm really interested in x2 here. So, it is gonna be 0 in differentiation, right?

4. Oct 4, 2009

### Fooze

Re: differentiate

I believe the point was to just give you an idea of how it would go, not do the exact problem for you.

However, yes... x2 is 0 since it's treated as a constant.

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