Results 1 to 3 of 3

Math Help - angle between the vectors

  1. #1
    Member
    Joined
    Oct 2007
    Posts
    134

    angle between the vectors

    Please check my work. Thank you.
    Find the dot product of the following vectors. Find the angle between the vectors.
    v=3i+2j, w=-4i-5j
    Dot product v x w=-12-10=-22
    cos of angle = -22/sq. root of 13 x sq. root of 41=-0.9529
    the angle = 17.64 degrees.

    Please tell me where I made a mistake, because when I graph the vectors, the angle looks like it is about 100 degrees.

    Thank you very much.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    skeeter's Avatar
    Joined
    Jun 2008
    From
    North Texas
    Posts
    11,621
    Thanks
    426
    Quote Originally Posted by oceanmd View Post
    Please check my work. Thank you.
    Find the dot product of the following vectors. Find the angle between the vectors.
    v=3i+2j, w=-4i-5j
    Dot product v x w=-12-10=-22
    cos of angle = -22/sq. root of 13 x sq. root of 41=-0.9529
    the angle = 17.64 degrees.

    Please tell me where I made a mistake, because when I graph the vectors, the angle looks like it is about 100 degrees.

    Thank you very much.
    a bit larger than 100 degrees, but it is obtuse ...

    \theta = \arccos(-0.9529) = 162.3^{\circ}

    inverse cosine of a negative value yields a quad II angle.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Oct 2007
    Posts
    134
    Thank you very much
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Vectors and an angle
    Posted in the Advanced Algebra Forum
    Replies: 2
    Last Post: April 6th 2011, 09:24 AM
  2. Angle between two vectors
    Posted in the Math Software Forum
    Replies: 6
    Last Post: November 22nd 2010, 07:40 AM
  3. Angle between Vectors
    Posted in the Advanced Algebra Forum
    Replies: 2
    Last Post: November 26th 2009, 07:44 AM
  4. Angle from 3d Vectors
    Posted in the Math Challenge Problems Forum
    Replies: 1
    Last Post: May 1st 2008, 06:49 AM
  5. Angle between vectors
    Posted in the Advanced Applied Math Forum
    Replies: 4
    Last Post: December 28th 2007, 08:30 AM

Search Tags


/mathhelpforum @mathhelpforum