integration by part?

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  • Alan
    Lv 7
    1 month ago
    Favorite Answer

    Integration by parts says 

    ∫,udv = uv -  ∫vdu  

    so pick u  and v    

    dv = (5)e^(5x) dx

    v = e^(5x)  

    u = (1/5) x

    du = 1/5   dx

    Into the formula 

     =   e^5x( x/5)  -  ∫ (1/5)e^(5x) dx

    = (1/5) xe^(5x)    - (1/25)e^(5x)     

    then add bounds   0 to 1  

    =         (1/5)e^(5)  -  (1/25)e^(5)   - ( 0 -  (1/25) )  

    =    (5/25 - 1/25) e^(5)   + 1/25   

    Exact answer 

    = (4/25)e^5   + 1/25       

    Approximate answer 

    = 23.78610546

  • 1 month ago

    After you've practiced these a while, substitution is u=x, dv=e^(ax)dx will practically write itself.  That leads to an easy du = 1, v = (1/a) e^(ax).

        ∫ x e^(ax) dx  =  ∫ u dv  =  uv - ∫ v du

                             =  x (1/a) e^(ax) - ∫ (1/a) e^(ax) dx

                             =  (x/a) e^(ax) - (1/a²) e^(ax) + C

                             =  (ax - 1)/a² * e^(ax) + C

    The rest should be easy, right?  Plug a=5, evaluate at x=0 and x=1, then subtract.

    For "by-parts" to work, look for something as u that will get simpler when you take the derivative.  Small positive powers of x are easy things to try for that.  You also want something left over in dv that you can easily integrate, and something like e^(ax) dx is a common choice.

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