*To*: mathgroup@smc.vnet.net*Subject*: [mg10932] Re: [mg10844] How do u go about doing this---?*From*: jpk@max.mpae.gwdg.de*Date*: Thu, 12 Feb 1998 20:16:19 -0500

> > Let f(x) = sin(x)+cos(y) > ----------------- > sin(x) - cos(y) > > Show that > (df/dx)^2 + (df/dy)^2 = > > 2+cos(2(x-y))+cos(2(x+y)) > ----------------------------------- > (sin(x)-cos(y))^4 > > > Thanks, > > L. > > > 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