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Difference between differentiating a function and an equation

  1. Aug 15, 2012 #1
    What is the difference? I always see differentiate a function but never an equation, a lot of exercises have y=blahblah which is an equation. Does it just mean that when you're asked to differentiate the equation (without using implicit), that it is satisfies the conditions for a function?
     
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  3. Aug 15, 2012 #2

    chiro

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    The difference is that the function case is a special case of the general case of implicit differentiation: your function is explicit with respect to the rest of the variables.

    It's basically akin to the difference of say d/dx(f) instead of say d/dx(f*x) where first is df/dx and the second is x*df/dx + f.

    Again its best if you think of a function as just another variable (this is it all it is) and that instead of the variable f being inter-twined where it can't be easily algebraically separated, it is explicit which means you can put f on one side and all the other variables on the other.
     
  4. Aug 15, 2012 #3

    HallsofIvy

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    The reason you only see "differentiate a function" is that is the way differentiation is defined. You don't differentiate an equation, you differentiate the functions on the two sides of the equals sign.
     
    Last edited: Aug 16, 2012
  5. Aug 16, 2012 #4
    ^ Ooh, ok, thank you!
     
  6. Aug 17, 2012 #5
    Only identities can be differentiated meaningfully. Equations are usually valid only for a select number of unknown values, and the "differentiated equation" may have a completely different set of roots. For example: [tex]3x = x^2 + 2[/tex]has roots [tex]x = 1, 2[/tex]However, differentiating, we have[tex]3 = 2x[/tex] where the root is [tex]x = \frac 3 2[/tex] This is why it rarely makes sense to differentiate equations (unless we are talking about functional equations).
     
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