# Derivative of a complex function using limit definition

• Feb 28th 2013, 07:10 AM
Ragnarok
Derivative of a complex function using limit definition
Find the derivative of $f(z)=5z^2-10z+8$ using the following definition:

$f'(z_0)=\lim_{z\to z_0}\frac{f(z)-f(z_0)}{z-z_0}$

I started with

$f'(z_0)=\lim_{z\to z_0}\frac{5z^2-10z+8-(5z_0^2-10z_0+8)}{z-z_0}$

which simplifies a bit to

$f'(z_0)=\lim_{z\to z_0}\frac{5z^2-10z-5z_0^2+10z_0}{z-z_0}$

but where do you go from there? Thanks!
• Feb 28th 2013, 07:38 AM
Plato
Re: Derivative of a complex function using limit definition
Quote:

Originally Posted by Ragnarok
Find the derivative of $f(z)=5z^2-10z+8$ using the following definition:

$f'(z_0)=\lim_{z\to z_0}\frac{f(z)-f(z_0)}{z-z_0}$

I started with

$f'(z_0)=\lim_{z\to z_0}\frac{5z^2-10z+8-(5z_0^2-10z_0+8)}{z-z_0}$

which simplifies a bit to

$f'(z_0)=\lim_{z\to z_0}\frac{5z^2-10z-5z_0^2+10z_0}{z-z_0}$

but where do you go from there? Thanks!

$\frac{5z^2-10z-5z_0^2+10z_0}{z-z_0}=\frac{5(z^2-z_0^2)-10(z-z_0)}{z-z_0}$
• Feb 28th 2013, 07:53 AM
hollywood
Re: Derivative of a complex function using limit definition
Try this:

$f'(z_0)=\lim_{z\to z_0}\frac{5z^2-5z_0^2-10z+10z_0}{z-z_0}$

$f'(z_0)=\lim_{z\to z_0}\frac{5(z+z_0)(z-z_0)-10(z-z_0)}{z-z_0}$

- Hollywood

Awww - Plato, you beat me!
• Feb 28th 2013, 07:55 AM
Ragnarok
Re: Derivative of a complex function using limit definition
Ah thank you! I forgot my difference of squares factorization.