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A. Find simpler, equivalent expressions for the following. Justify your answers.
(a) sin(180 + è) (b) cos(180 + è) (c) tan(180 + è)
B. Show that there are at least two ways to calculate the angle formed by the vectors
[cos 19, sin 19] and [cos 54, sin 54].

  • Trig -

    I will do a)
    sin(180 + è)
    = sin180cos è + cos180sin è
    = 0 + (-1)sin è
    = -sin è

    for the other two, you will have to know the expansion for cos(180+ è) and tan(180+ è)

    B) [cos 19, sin 19]•[cos 54, sin 54] = |[cos 19, sin 19]||[cos 54, sin 54]|cos Ø, where Ø is the angle between
    cos19cos54 + sin19sin54 = 1x1cosØ
    cos(19-54) = cosØ
    Ø = |19-54|
    = 35°

    second way: vector [cos 19, sin 19] makes an angle P with the x-axis such that tan P = sin19/cos19
    tan P = tan 19
    P = 19

    similarly the second vector makes an angle of 54° with the x-axis
    so the angle between them is 35°

  • Trig -

    A. use sum of angles formulae:


    for tan(180+α)
    use (tan A + tan B)/(1 - (tan A)(tan B))

    Since both vectors are unit vectors, the cosine of the included angle is simply the dot product of A(cos(a),sin(a)) and B(cos(b),sin(b)):
    =(cos(a)cos(b)+sin(a)sin(b))/(|A| |B|)
    =(cos(a-b))/(|1| |1|)
    therefore θ=a-b or -(a-b)

    The magnitude of the cross product of the two vectors represent the positive area of a parallelogram formed by the two vectors as adjacent sides, namely,
    Area=ABsin(θ)=|A x B|

    Also, since the terminal points of the vectors A and B represent points on the unit circle at angles a and b from the x-axis, we conclude that the angle between the vectors is |a-b|.

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