# Physical Significance of d(f(x,y))

• Apr 12th 2013, 06:55 AM
rickrishav
Physical Significance of d(f(x,y))
What is the physical meaning of d(x2y2)?

For example,
d(x2) = 2x dx
This can be derived as,

d(x2)/dx = 2x
d(x2) = 2x dx

Here dx can be thought of Delta x, rather than thinking d/dx as an operation. So, small change in x2 is the product of "two times current value of x" and small change in x.

What will be the similar deduction methodology for d(x2y2) = 2xy2dx + 2yx2dy?

Thanks a lot.
• Apr 12th 2013, 09:16 AM
HallsofIvy
Re: Physical Significance of d(f(x,y))
Given $\displaystyle f(x,y)= x^2y^2$ then $\displaystyle grad f= \nabla f= 2xy^2\vec{i}+ 2x^2y\vec{j}$ is a vector pointing in the direction of fastest increase of f with length equal to the rate of change of f in that direction. We can think of "$\displaystyle dx\vec{i}+ dy\vec{j}$" as a vector a "small" distance, dx, parallel to the x-direction, and a "small distance", dy, parallel to the y-axis. So $\displaystyle df= 2xy^2dx+ 2x^2ydy$ is the dot product of those two vectors, the rate of change of f along this "infinitesmal" vector.
• Apr 12th 2013, 06:32 PM
rickrishav
Re: Physical Significance of d(f(x,y))