Suppose that f is a differentiable function of a single variable and is defined by .
Show that satisfies the partial differential equation .
Given that for all , find a formula for .
I'm pretty sure im supposed to use the chain rule for this but i dont know how to apply it. Can someone care to explain?
Thanks for the useful reply. I'm just a little confused with when to use the normal d's and the curly d's. Say for example, we have where is a constant and we have to find . Is the following working correct?
Also, how come the chain rule works with both normal d's and curley d's. Is there some sort of proof for the result dealing with more than one variable. Sorry, im new to this topic so any help would be appreciated.
Basically, anytime you have a function that is defined by a single variable ( in this case), you would take what is called the "total" derivative which is
But, when dealing with a function of more than one variable, i.e., , you must use the partial derivatives which are
You probably understand this, but it may seem odd to use both notations in the same context. For example, the problem you gave was
Notice that when you make the substitution, you can write the function as which is entirely defined by a single variable . So:
Therefore, is now a single variable function, so when you take a derivative of with respect to , you would write it as .
But when you take a derivative of with respect to , you would write it as .