Results 1 to 8 of 8

Math Help - Vector..again

  1. #1
    Member
    Joined
    Apr 2013
    From
    Mahinog
    Posts
    89
    Thanks
    5

    Vector..again

    The angle between vectors a and b is 45. If |a| =5 units and |b| =4 units, find the value of |a-b| and of |a+b|
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,648
    Thanks
    1596
    Awards
    1

    Re: Vector..again

    Quote Originally Posted by Trefoil2727 View Post
    The angle between vectors a and b is 45. If |a| =5 units and |b| =4 units, find the value of |a-b| and of |a+b|
    Think of \vec{a}~\&~\vec{b} as adjacent sides of a parallelogram.
    Then \vec{a}-\vec{b} is the diagonal opposite the angle between them.
    \vec{a}+\vec{b} is the diagonal opposite the angle adjacent to the angle between them.

    Now use the law of cosines.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor

    Joined
    Apr 2005
    Posts
    15,693
    Thanks
    1466

    Re: Vector..again

    I hope that you know that |a- b|^2= |a|^2- 2a\cdot b+ |b|^2 and |a+ b|^2= |a|^2+ 2a\cdot b+ |b|^2.

    And you should know that a\cdot b= |a||b|cos(\theta) where \theta is the angle between a and b.

    I see Plato got in just ahead of me. Different points of view about the same thing.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Member
    Joined
    Apr 2013
    From
    Mahinog
    Posts
    89
    Thanks
    5

    Re: Vector..again

    but why a.b=|a||b|cos θ?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,648
    Thanks
    1596
    Awards
    1

    Re: Vector..again

    Quote Originally Posted by Trefoil2727 View Post
    but why a.b=|a||b|cos θ?
    If you have to ask, then you are in no way ready to do this question.
    Why then were you asked to work this question?
    Follow Math Help Forum on Facebook and Google+

  6. #6
    Member
    Joined
    Apr 2013
    From
    Mahinog
    Posts
    89
    Thanks
    5

    Re: Vector..again

    still can't find a+b..
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Member
    Joined
    Apr 2013
    From
    Mahinog
    Posts
    89
    Thanks
    5

    Re: Vector..again

    I don't know, maybe my teacher likes to make his students suffer
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Member
    Joined
    Apr 2013
    From
    Mahinog
    Posts
    89
    Thanks
    5

    Re: Vector..again

    hah, got it! thanks!
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 0
    Last Post: December 3rd 2012, 01:25 PM
  2. Replies: 0
    Last Post: July 24th 2012, 02:41 AM
  3. Replies: 8
    Last Post: January 11th 2011, 10:17 AM
  4. Replies: 2
    Last Post: January 27th 2010, 08:23 PM
  5. Replies: 2
    Last Post: October 5th 2009, 03:25 PM

Search Tags


/mathhelpforum @mathhelpforum