Thursday, 30 June 2016

multivariable calculus - Finding the partial derivatives of this function




Let gC1(R) be a real valued function and f defined by f(u,v,w)=vug(w2+s)ds where u,v,wR 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(w2s). When you compute the partial derivatives with rispect to u,v the variable w is fixed, hence you can think h as a function RR.




So
uvuh(s)ds=uuvh(s)ds=h(u)
since v is considered a constant. In the same way you get
vvuh(s)ds=h(v)
While for the third partial derivative you need to exchange the derivative with the integral sign, so you get
wvug(w2s)ds=vuwg(w2s)ds=vug(w2s)2w ds


No comments:

Post a Comment

real analysis - How to find limhrightarrow0fracsin(ha)h

How to find lim without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...