Thread: Partial Derrivatives Assignment problem

Originally Posted by elbarto

Hi,
I am an engineering student undertaking a subject called "mathematics for scientist and engineers". I was hopeing someone here could help me better understand this question as I really have no idea on how to even begin to solve it.
Thank You
Elbarto
[IMG]file:///C:/DOCUME%7E1/Barto/LOCALS%7E1/Temp/moz-screenshot.jpg[/IMG]

z = f(x^2 + y^2)

When we take the partial derivative of z with respect to y we take an "ordinary" derivative, but assume that y is constant. Thus:
(del)z/(del)x = f'(x^2 + y^2) * (2x)
where (del)z/(del)x represents the partial derivative and f'(u) represents the derivative of f(u) with respect to its argument. This is nothing more than the chain rule where we have set y equal to a constant.

Similarly:
(del)z/(del)y = f'(x^2 + y^2) * (2y)

So:
y * (del)z/(del) - x * (del)z/(del)y

= y* f'(x^2 + y^2) * (2x) - x * f'(x^2 + y^2) * 2y

= 2xy* f'(x^2 + y^2) - 2xy * f'(x^2 + y^2) = 0
as advertised.

-Dan

Hi Dan,
Thank you very much for your fast reply. Makes so much more sense after someone explains it. I now understand what they mean when they say f is a differentiable function of one variable. I cant thank you enough.
Regards Elbarto