Re: How do u go about doing this---?
- 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