# Question

• Feb 22nd 2013, 01:20 PM
landmark
Question
Looking at some of the older stuff and for ∫1/10(x)(6x^2-15) dx became 1/120 ∫12x(6x^2-15) dx, how was the 1/120 ∫12x arrived at?
• Feb 22nd 2013, 01:43 PM
coffee2000
Re: Question
1/10 = 1/120 * 12
and we know that you can move a constant outside of the integral
• Feb 22nd 2013, 01:47 PM
landmark
Re: Question
I can see that but why 12?
• Feb 22nd 2013, 01:49 PM
coffee2000
Re: Question
probably to make u-substitution more straightforward

i.e.
u=6x^2-15
du=12x dx
• Feb 22nd 2013, 02:03 PM
landmark
Re: Question
Not sure if I quite get it but thanks
• Feb 22nd 2013, 02:16 PM
coffee2000
Re: Question
EDIT:
1/120 ∫12x(6x^2-15) dx
=1/120
∫ 72x^3-180x dx

I'm sorry i forgot that it was possible that you didn't cover u-substitution, which is just a method of integration and all you had to do was multiply it out and integrate the polynomial! sorry! :)
and 1/120 was probably to avoid fractions during integration
• Feb 22nd 2013, 11:55 PM
landmark
Re: Question
Thanks for that I have to get used to such manipulation, I have never been on one of these forums before and can not figure out how to do a smiley face
• Feb 23rd 2013, 06:43 AM
topsquark
Re: Question
Quote:

Originally Posted by landmark
Thanks for that I have to get used to such manipulation, I have never been on one of these forums before and can not figure out how to do a smiley face

The usual way: :) = : + )

-Dan