*To*: mathgroup@smc.vnet.net*Subject*: [mg10957] Re: [mg10932] Re: [mg10844] How do u go about doing this---?*From*: jpk@max.mpae.gwdg.de*Date*: Sat, 14 Feb 1998 00:53:27 -0500

Yes, Sorry, Thanks Jens > Oops Jens, > you overlooked the fact that he wanted the sum of squares of the first > derivative, not the sum of the second derivatives. > Happens to the best of us, at least some of the time. I goof myself now & > then, I know the feeling. But *do* go on replying. The least we, non-guru's, > can do is relieve some of the workload of the "Abbotts & Hintons", letting > them treat the stuff we can't. > > with sympathy > > wouter. > > >> Show that > >> (df/dx)^2 + (df/dy)^2 = > >Hi, it can't be shown because with > >f[x,y]->(Sin[x]+Cos[y])/(Sin[x]-Cos[y]) > > > >one gets for > > > >D[f[x,y],{x,2}]+D[f[x,y],{y,2}]= > > > >(-4*(Cos[x]^2*Cos[y] + Sin[x]*Sin[y]^2))/(Cos[y] - Sin[x])^3 > > > >what is clearly different from Your "result" > > > >Hope that helps > > Jens > > > > > > NV Vandemoortele Coordination Center > Oils & Fats Applied Research > Prins Albertlaan 79 > Postbus 40 > B-8870 Izegem (Belgium) > Tel: +/32/51/33 21 11 > Fax: +/32/51/33 21 75 > vdmcc@vandemoortele.be >