Notation: df/dx in x=a

[Function]

Hello everyone

 

Pretty stupid: I was wondering how you can write "the derivative of a function f(x) in a" (to use in proofs), using [math]\frac{d}{dx}[/math], not just [math]f'(a)[/math]:

 

[math]\frac{df}{dx}_{x=a}[/math]?

 

[math]\frac{d}{dx}f(a)[/math]?

 

[math]\frac{df}{dx}(a)[/math]?

 

[math]\frac{df}{dx}_a[/math]?

 

Something else? Can someone help me on this one?

 

Thanks.

 

Function.

Edited December 2, 2013 by Function

[ajb]

At the point a

 

[math]\left. \frac{d}{dx}\right|_{a} f[/math] or [math]\left. \frac{df}{dx}\right|_{a}[/math] are common.

 

You want to be sure that no-one will think that your function is a function of a and not x.

[Function]

At the point a

 

[math]\left. \frac{d}{dx}\right|_{a} f[/math] or [math]\left. \frac{df}{dx}\right|_{a}[/math] are common.

 

You want to be sure that no-one will think that your function is a function of a and not x.

Thanks.

 

P.S. What's the function of "\left." in your TeX code?

[ajb]

P.S. What's the function of "\left." in your TeX code?

It makes sure that the line on the right is tall enough.