## anonymous one year ago Would I have to use the 'u' substitution method to solve this integral?

1. anonymous

2. nincompoop

can you try without u-sub?

3. Astrophysics

Ye, u = lnx

4. anonymous

@nincompoop lol no clue

5. Astrophysics

You can pull the -9 out as it's just a constant

6. anonymous

Ohh right! I always forget that! >.<

7. Astrophysics

$-9 \int\limits \frac{ \left( lnx \right)^2 }{ x } dx$ u = lnx

8. anonymous

then dx = (1/x) du

9. Astrophysics

$du = \frac{ 1 }{ x }dx \implies dx= xdu$

10. anonymous

Isn't du/dx = 1/x ?

11. Empty

Yeah you're both right :P

12. anonymous

Ohh ahah right! I've lost my mind xD

13. Astrophysics

This anonymous user who keeps joining makes me lose connection

14. anonymous

kk so then.... $-9\int\limits \frac{u^2}{x}(\frac{1}{x})du = -9\int\limits \frac{ u^2 }{ x^2 }du$ right?

15. anonymous

Which anon user? :O

16. Astrophysics

Oh it shows up for me then I refresh haha...but when you do a u sub, you should not have any x's in your integrand

17. Astrophysics

Well, unless you're just subbing in for du, then the x's should get cancelled out

18. Astrophysics

$-9 \int\limits \frac{ u^2 }{ x }xdu$ right/

19. anonymous

Wait what just happened? o.o

20. Astrophysics

|dw:1437192143650:dw|

21. anonymous

Right, followed till here

22. Astrophysics

|dw:1437192220267:dw| sorry I forgot it was squared, so you can just edit that, but this is the method, it should make more sense since I wrote it out as well haha.

23. anonymous

Ohhhh OK didn't notice that! Yep, makes sense!

24. Astrophysics

Alright cool :D

25. Astrophysics

BTW don't forget the +C

26. anonymous

True true true!!! XD Haha was about to forget! Lol thanks so much you math genius! :D

27. Astrophysics

http://www.wolframalpha.com/input/?i=integrate+%28-9%28lnx%29%5E2%29%2Fx+dx is what you should get even though you don't need wolfram for this xD

28. anonymous

SO final answer would be $-9(\frac{ (lnx)^3 }{ 3 })$ ... i think :P haha

29. Astrophysics

Yup!

30. Astrophysics

+C

31. anonymous

Hahaha oh gee m bad XD yes +C

32. anonymous

my*

33. Astrophysics

$\huge \checkmark$

34. anonymous

Niiiiiiice how'd you do that?? :O

35. Astrophysics

\checkmark lol

36. anonymous

lemme try XD

37. anonymous

\checkmark

38. anonymous

Heyyyyy it aint working :P LOL

39. Astrophysics

No no, I meant for latex, so you can put it in the equation bar

40. anonymous

$\checkmark$

41. anonymous

Yours was bigger -.-

42. anonymous

Hahahaha

43. Astrophysics

Yay, you can also right click on peoples latex and go on "show math as" and it will show what they used, so I also put \huge checkmark

44. Astrophysics

the \huge makes it bigger :P

45. anonymous

Ohhhhh :P smart ;P

46. anonymous

can we also put in \super huge checkmark ?? XP would that work?? XD

47. Astrophysics

Haha no, but you can experiment with it, large, tiny xD

48. Astrophysics

$\tiny \text{astrophysics}$

49. anonymous

Hahaha thats awesome!! $\Large BOO!$ scare ya? :P

50. Astrophysics

|dw:1437193445973:dw| ahhhh

51. Astrophysics

lol

52. anonymous

Hahaha LOL XD

53. anonymous

Arighty better close this question before the mathematicians here get pissed at me xD lol

54. Astrophysics

Haha, don't have to worry about them ;)

55. anonymous

Hahaha *thumbs up*