# Help with an integral

1. Jan 24, 2007

### who's_next?

i have to integrate to find the electric potential due to a disk of charge with an uneven charge distribution. i was able to set up the intergral but i cant solve it. should i do integration by parts (i tried it) or use something else. just give me a hint

int ( r^3/[(x^2 + r^2)]^.5) dr (x is a constant)

thanks

2. Jan 25, 2007

### dextercioby

Make the substitution

$$r=x\sinh t$$

Daniel.

3. Jan 25, 2007

### Gib Z

>.< I don't know Daniels way, Id go with trigonometric substitution, tan theta = r/x. Draw a triangle, you see that the hypotnuse is your denominator. Just solve for everything and sub them in, it reduces to $$x^3\int \tan^3 \theta \sec \theta d\theta$$.

4. Jan 25, 2007

### who's_next?

yea i figured it out. i did uv substitution, takeing u=r^2 and dx= r/(r^2+x^2) ^9.5)

thank you for everyone who gave advice. :D