*To*: mathgroup@smc.vnet.net*Subject*: [mg10898] RE: [mg10844] How do u go about doing this---?*From*: R Finley <trfin@umsmed.edu>*Date*: Thu, 12 Feb 1998 20:15:33 -0500

L, f[x_,y_] := (Sin[x] + Cos[y])/(Sin[x] - Cos[y]) Simplify[D[f[x,y],x]^2 + D[f[x,y],y]^2] Gives you the answer you desire. RF -----Original Message----- From: McAllister [SMTP:lmcallister@email.msn.com] To: mathgroup@smc.vnet.net Sent: Tuesday, February 10, 1998 8:02 PM To: mathgroup@smc.vnet.net Subject: [mg10844] How do u go about doing this---? 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.