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Homework Help: Integration of linear functions

  1. Sep 23, 2012 #1
    1. The problem statement, all variables and given/known data
    If i integrate (2x+5) using the simplest rule, i get x2+5x+c, however if i use the linear function method, i would get (2x+5)^2/4 +c so simplifying it i get (4x^2+20x+25)/4 +c=x^2+5x+25/4+c. So does it mean when i use the two different methods my constant,c is different?


    2. Relevant equations
    linear integration rule

    3. The attempt at a solution
    So does it mean when i use the two different methods my constant,c is different?
     
  2. jcsd
  3. Sep 23, 2012 #2
    Yes, there's no meaningful difference there.
     
  4. Sep 23, 2012 #3
    thanks!
     
  5. Sep 23, 2012 #4

    HallsofIvy

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    Science Advisor

    C is an arbitrary constant. It doesn't matter whether you use "c" or "25/4+ c" it still represents some arbitrary constant.

    Suppose you were given that dy/dx= 2x+ 5 and that y(0)= 1. Clearly y is the integral of 2x+ 5 so, integrating the first way, you would get y(x)= x2+ 5x+ c and then the addtional condition, that y(0)= 1, becomes y(0)= 02+ 5(0)+ c= c= 1 so you answer is y(x)= x2+ 5x+ 1.

    Integrating the second way, y(x)= x2+ 5x+ 25/4+ c so that y(0)= 02+ 5(0)+ 25/4+ c= 25/4+ x= 1 so that c= 1- 25/4= -21/4. But that means that y(x)= x2+ 5x+ 25/4- 21/4= x2+ 5x+ 4/4= x2+ 5x+ 1 just as before.
     
  6. Sep 23, 2012 #5

    Mentallic

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    Homework Helper

    If I ever use a method of integration that gives me some constant such as +1 but then I also have a +c added on the end, I just merge them together to make it +c2 or +k or if the teacher isn't strict / doesn't care I'll just get rid of the 1.
     
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