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RE: Partial derivatives for implicit functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg98333] RE: [mg98303] Partial derivatives for implicit functions
  • From: "David Park" <djmpark at comcast.net>
  • Date: Mon, 6 Apr 2009 05:02:19 -0400 (EDT)
  • References: <6934246.1238929226790.JavaMail.root@n11>

Srikanth,

Use implicit differentiation. Copy and evaluate the following in one cell.

Print["Find dy/dx for the equation:"]
eqn1 = 2 y == x^2 + Sin[y]
Print["To calculate dy/dx implicitly write equation with y as a \
function of x."]
eqn2 = eqn1 /. y -> y[x]
Print["Differentiate with respect to x"]
step1 = D[eqn, x]
Print["Solve the equation for y'[x]."]
step2 = Equal @@ Part[Solve[step1, y'[x]], 1, 1]
Print["Change y[x] back to y."]
step3 = step2 /. y[x] -> y
Print["Get rid of the extra minus signs."]
MapAt[Minus, step3, {{2, 1}, {2, 3, 1}}]

If you have Presentations you could also get rid of the minus signs with:

Needs["Presentations`Master`"]

MapAt[MultiplyByOne[-1], step3, 2]

which multiplies numerator and denominator by -1.


David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/  


From: Srikanth [mailto:skt at xdtech.com] 


Hi
I have a function f(x,y,z)=0. I would like to find dx/dy and dx/dz
(where I am using d for partial derivatives, I'm not sure how to get
the actual symbol in this edit box), where both would be functions of
x, y and z. Unfortunately, I cannot get an explicit solution x=g(y,z).
Any ideas on how I'd go about solving this?
Thanks
Srikanth




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