Can somebody please help me with this math problem with intergrals? I really don´t know how to solve this and I´ve been sitting with this for days now.  I´ve tried to solve it with partly...

Can somebody please help me with this math problem with intergrals? 
I really don´t know how to solve this and I´ve been sitting with this for days now. 

I´ve tried to solve it with partly intergration where I choose the first part to be f(x) and the other part to be g´(x) and tried to use the product rule to solve them, but it´s becoming even more complex as I go by...

I would be so happy if I can get some help or tips on how to solve this.

Asked on by pham1403

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted on

The integral `int (1/sqrt 2)V_0*e^(-kt)*sqrt(1+e^(-kt)) dt` has to be determined.

Let `1+e^(-kt) = y`

`dy/dt = -k*e^(-kt)`

=> `e^(-kt) dt = (-1/k)dy`

`int (1/sqrt 2)V_0*e^(-kt)*sqrt(1+e^(-kt)) dt`

=> `int (1/sqrt 2)V_0*(-1/k)sqrt(y) dy`

=> `-V_0/(sqrt 2*k)int sqrt(y) dy`

=> `-V_0/(sqrt 2*k)y^(3/2)/(3/2)`

=> `(-2*V_0)/(3*sqrt 2*k)y^(3/2)`

Substitute `y = 1+e^(-kt)`

=> `(-2*V_0)/(3*sqrt 2*k)(1+e^(-kt))^(3/2)`

The required integral `int (1/sqrt 2)V_0*e^(-kt)*sqrt(1+e^(-kt)) dt = (-2*V_0)/(3*sqrt 2*k)(1+e^(-kt))^(3/2)`

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pham1403 | (Level 1) eNoter

Posted on

Wow thank you so much, it totally make sense on how to solve this.

I´ve tried using your method before, but I failed to see what the derivative of "1+e^(-kt)", but now I understand this.

Really thank you so much! =)

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