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Matlab Symbolic Math Toolbox

  1. Sep 12, 2011 #1
    Does anybody know much about how this works?

    I can't understand why it doesn't give me an answer for definite integrals sometimes... e.g. in Matlab R2011a:

    syms x a;
    int(x/sqrt(x^2+a^2))

    gives the answer (a^2 + x^2)^(1/2) as expected.

    But int(x/sqrt(x^2+a^2),0,10) gives - Warning: Explicit integral could not be found.

    This doesn't really make any sense as the answer should be (a^2 + 10^2)^(1/2) - a. Is there some way to understand this strange behavior?
     
  2. jcsd
  3. Sep 12, 2011 #2
    In this case you have to assume that a is real. Even then the solution will be plus/minus a given that we dont know the sign of a.

    evalin(symengine,'assume(a in R_)')
    syms x a
    int(x/sqrt(x^2+a^2),0,10);

    ans =

    (a^2 + 100)^(1/2) - abs(a)
     
  4. Sep 12, 2011 #3
    Ah, thank you! I understand!
     
  5. Sep 12, 2011 #4

    jhae2.718

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    Gold Member

    Another way to declare a real (the code below will actually declare both x and a real, but the example stands) is:
    Code (Text):

    syms [COLOR="Magenta"]x a real[/COLOR]
     
    I find it easier to remember that way.
     
  6. Sep 12, 2011 #5
    that's perfect, thank you!
     
  7. Sep 12, 2011 #6

    jhae2.718

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    Gold Member

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