YACalculusT: How do you use u*du properly in this integral...

[Ricemarine]

***In this case, since that limit sign is just a big squiggle, i'll use S as the squiggle

limit from -2 to 2 (3u+1)^2 du.

So, first thing, decide on U

u=3u+1

Then solve for du/dx, or in this case for random variables, dn/du
[/quote]dn/du = 3, ----> dn/3 = u*dx[/quote]
Here's where I got lost...

1/3 S u^2 *1/3du ----> 1/3 S (3u+1) * du

I know since U is a new variable, the new limits would be 1 and 7...

But as you can see, I am very very lost as to how to go about this... My mat124 calculus teacher told us to scratch the concept in the book and follow his. But this mat125 teacher tells us to go by the book, so I am having a bit of trouble on to how to use U *du properly.

~Thanks for your help.

 

[Fenixgoon]

you could just expand the polynomial and integrate.... (if im reading it right)

 

[Ricemarine]

Originally posted by: Fenixgoon
you could just expand the polynomial and integrate.... (if im reading it right)

I'm not following

 

[Fenixgoon]

Originally posted by: Ricemarine

Originally posted by: Fenixgoon
you could just expand the polynomial and integrate.... (if im reading it right)

I'm not following

(3u+1)^2 = int (9u^2 + 6u +1)du from -2 to 2

 

[PhlashFoto]

if you expand the polynomial out and u-sub it then, it then turns into a limit -2 to +2; Integral udu problem.

Dont forget to change your limits if need be.

 

[Fenixgoon]

Originally posted by: ELopes580
if you expand the polynomial out and u-sub it then, it then turns into a limit -2 to +2; Integral udu problem.

Dont forget to change your limits if need be.

why u sub after expanding? i dont see why it's necessary

 

[Ricemarine]

Originally posted by: ELopes580
if you expand the polynomial out and u-sub it then, it then turns into a limit -2 to +2; Integral udu problem.

Dont forget to change your limits if need be.

Hmm, I don't understand why if you u sub it then the limit gets reversed...

But in anycase, fenixgoon was correct that if you expanded the polynomial, and then plugged in the limits from there, it would equal the answer, 52.

 

[LS20]

how do u*du that thing that u*du so well