Dot product

  • Thread starter PiRsq
  • Start date
  • #1
110
0
How do you expand and simplify this one?


(2u+v).(u-2v)
=2u.u+2u.(-2v)+v.u+v.(-2v)

Where u and v are vectors and the "." is the dot. I did some but after this im lost, how can I continue?
 

Answers and Replies

  • #2
HallsofIvy
Science Advisor
Homework Helper
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I don't see what your problem is. You have done
(2u+v).(u-2v)=2u.u+2u.(-2v)+v.u+v.(-2v) correctly. Now just
combine 2u.(-2v)= -4 u.v and v.u which is also u.v:
This is 2(u.u)- 3 u.v- 2(v.v)

You can also use the fact that |u|= sqrt(u.u) so that
u.u= |u|2 and v.v= |v|2 so that
(2u+v).(u-2v)= |u|2- 3u.v- 2|v|2.
 
  • #3
110
0
Got it, thanks :smile:
 

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