# Need help on an Integral

1. Feb 23, 2010

### juice34

1. The problem statement, all variables and given/known data
I am having trouble finding the right solution to this integral it is of the form int(1/x)dx=lnx+c but apparently im doing it wrong because of the other factors in the denominator
integral(dx/[(k1+k2)x-k2*y]

2. Relevant equations

3. The attempt at a solution
i get (1/(k1+k2))(ln(x/xi))-(x-xi)/k2*y but apparently the answer is
(1/(k2+k2))ln[((k1+k2)x-k2*xi)/((k1+k2)xi-k2*xi)]
so what am i doing wrong

2. Feb 23, 2010

### juice34

Re: Integral

please note that i evalued the integrals for both from xi to x also

3. Feb 23, 2010

### Staff: Mentor

Re: Integral

$$\int_{x_i}^x \frac{dt}{(k_1 + k_2)t - k_2 y}$$

It's unusual to have a variable as a limit of integration that is also the dummy variable of the integrand, so I switched the dummy variable to t.

To do this integral, use u = (k_1 + k_2)t - k_2*y. Then du = (k_1 + k_2)dt. Can you finish this?

4. Feb 23, 2010

### juice34

Re: Integral

Thank you mark I understand now, you are a great help!!!

5. Feb 23, 2010

Re: Integral

Aw, shucks!