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Homework Help: Vectors. Triangle Angle between vector

  1. Mar 4, 2013 #1
    1. The problem statement, all variables and given/known data
    Consider the points A(1,2) B(-2,-1) and C(3,-2) on the Cartesian plane.

    1. Find the length of the segments AB and BC

    2. Find the angle of the triangle at the vertex B

    2. Relevant equations

    Cosθ=(AB*BC)/|AB| |BC| ???

    3. The attempt at a solution

    The length of the segments; AB= <-3,-3> |AB|=√(18)
    BC =<5,-1> |BC|=√(26)

    So therefore the angle at point B is θ=arccos(((-3)(5)) (dot) ((-1)(-3))) / (√(18*26))

    θ=arccos(-18 / √(468))

    is this correct?

    I am assuming that the dot product would be the (x1)(x2) + (y1)(y2)
  2. jcsd
  3. Mar 4, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper

    The idea looks right, but (-3)(5)+(-1)(-3) isn't -18.
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