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Integration (fromula of reduction)

  1. Aug 17, 2013 #1
    Referring to the paint document. Could they have easily changed m into m + 2 or m + 1.5? Or does it have to contain the variable m and n?
     

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  3. Aug 17, 2013 #2
    Also, in the original equation (Equation 1 of the paint document) why did they chose the power of x to be (m-1)? Why not just leave it as m?

    Therefore we would get the equation

    ∫xmXpdx = Xpxm+1/(m+1) - bnp/(m+1)∫xm+n + 1Xp-1dx

    and therefore if we wanted to solve for the integral of say x2√(1+x) then the value of m is 2 and the value of n is 1 and the value of p is 1/2

    which saves you a step in solving for m: m-1 = 2 and m = 3, if we used the equation that they started with which was

    ∫xmXpdx = Xpxm+1/(m+1) - bnp/(m+1)∫xm+n + 1Xp-1dx where m is equal to m-1
     
  4. Aug 17, 2013 #3

    haruspex

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    Not sure I understand your question.
    You can make any of those substitutions as long as you are consistent. They chose m-n because it gave the form they were after on the LHS.
    Wrt m v. m-1, using m-1 on the LHS of (1) makes the equation as a whole slightly simpler.
     
  5. Aug 18, 2013 #4
    Say I was asked to solve the integral of x2/(x^2 + 1)1/2

    then I have:

    ∫x2/(x^2 + 1)1/2dx = ∫d{(x2+1)}1/2/dx*x*dx
    =(x2+1)1/2*x - ∫(x2+1)dx.

    On the LHS can I change the power of x in the numerator from 2 to 2 - m so long as I make the correct changes on RHS?

    Because that is what they essentially did
     
  6. Aug 18, 2013 #5

    haruspex

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    No, changing a constant to a variable is rather more drastic. You might have made use of specific attributes of the constant in arriving at the equation. All they did was a change of variable, but using the same name for the new variable.
     
  7. Aug 18, 2013 #6

    Ray Vickson

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    No, it is not what they did---and you cannot do it, either. 2 is 2 and you cannot re-write it as something else.

    In expanded form, what they did was, essentially, to first re-name m as m'-n, and rename p as p'+1, then remove the primes so that they can write m instead of m' and p instead of p'. They are not actually changing anything; they are simply re-naming things.
     
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