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I'm attempting to differentiate an equation in the form

D[sqrt((2*(((a*b*c+Pi*d*e^2+Pi*f*g^2+h*i*j+Pi*k*l^2)/(a*b*c+Pi*d*e^2+Pi*f*g^2))-1)*m)/(n^2 - o^2)/p), a]

in order to do an error propagation analysis. So I need to differentiate it against a, against b, against c and so on.

All my values will be positive (they reflect physical dimensions of my design) and the value of n will always be greater than o.

Is there a way to define all my variables as real and positive before I differentiate? And to define n greater than o?

The best I've found is $Assumptions = _Symbol [Element] Reals but this only gets me part of the way.

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assume = (And @@ 
     Thread[{a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p} > 0]) && n > o;

Note that when you state that a variable is positive then it is automatically also real. And for $Assumptions or Assuming to have an effect, you must use a function that takes the option Assumptions (e.g., Simplify).

Assuming[assume, Element[a, Reals] // Simplify]

(* True *)

expr = Assuming[assume, 
  D[Sqrt[(2*(((a*b*c + Pi*d*e^2 + Pi*f*g^2 + h*i*j + Pi*k*l^2)/(a*b*c + 
               Pi*d*e^2 + Pi*f*g^2)) - 1)*m)/(n^2 - o^2)/p], a] // Simplify]

enter image description here

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This is making the assumption that a is Real and positive, and then it calculates the derivative. But the result is zero.

Assuming[ a \[Element] Reals && a > 0, 
 D[sqrt ((2*(((abc + Pide^2 + Pifg^2 + hij + Pikl^2)/(abc + Pide^2 + 
              Pifg^2)) - 1)*m)/(n^2 - o^2)/p), a]]

Hope it helps.

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  • $\begingroup$ Thank you. I think your approach might work for me. I modified my equation to give the following....do you think this makes sense? Assuming[{a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p} \[Element] Reals && {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p} > 0, D[((2*(((a*b*c + Pi*d*e^2 + Pi*f*g^2 + h*i*j + Pi*k*l^2)/(a*b*c + Pi*d*e^2 + Pi*f*g^2)) - 1)*m)/(n^2 - o^2)/p)^0.5, a]] $\endgroup$ – DeltaJ Mar 16 '18 at 12:07
  • $\begingroup$ Hi @DeltaJ I think it would do the trick. If not, then you can use the following. a [Element] Reals && a > 0 && b [Element] Reals && b > 0 and so on for the rest of the letters. Hope this helps. Cheers!!! $\endgroup$ – A_user_with_NoName Mar 16 '18 at 12:32
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    $\begingroup$ Appreciate it thank you ;-) $\endgroup$ – DeltaJ Mar 16 '18 at 13:33

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