Let g∈C1(R) be a real valued function and f defined by f(u,v,w)=∫vug(w2+√s)ds where u,v,w∈R and u,v>0.
Find all partial derivatives.
I'm not sure how to attempt this problem. I assume if it were a function of two variables, say something like f(x,w)=∫x0g(w2+√s)ds then for example the partial derivative with respect to x would just be g(w2+√x) (is that true?).
Anyhow, some hint or strategy would be very welcomed.
Answer
Call h(s)=g(w2−√s). When you compute the partial derivatives with rispect to u,v the variable w is fixed, hence you can think h as a function R⟶R.
So
∂∂u∫vuh(s)ds=−∂∂u∫uvh(s)ds=−h(u)
since v is considered a constant. In the same way you get
∂∂v∫vuh(s)ds=h(v)
While for the third partial derivative you need to exchange the derivative with the integral sign, so you get
∂∂w∫vug(w2−√s)ds=∫vu∂∂wg(w2−√s)ds=∫vug′(w2−√s)2w ds
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