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Math Help - Vector from angle and length

  1. #1
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    Vector from angle and length

    Hello, I need a little help again, I have been working on this for a couple hours, and I am stuck.

    In my textbook I have this problem:
    Find the vector that has a length of 5 and an angle of 220 with positive x axis on the form ai+bj

    My best guess so far has been this:
    Cos(220)*5=-cos(40)*5 (I have not been able to simplify this more)
    Sin(220)*5=-sin(40)*5

    and written it as this:
    -(cos(40)5)i-(sin(40)5)j

    Now i controlled this by checking this:
    \sqrt{((sin(220)5)^2+(cos(220)5)^2} =5

    But the answer provided in the back of the textbook is this:

    -\frac{5\sqrt{3}}{2}i-\frac{5}{2}j

    My question is:
    What am I doing wrong? I am completely stuck.
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  2. #2
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    Re: Vector from angle and length

    Quote Originally Posted by mariusg View Post
    In my textbook I have this problem:
    Find the vector that has a length of 5 and an angle of 220 with positive x axis on the form ai+bj
    My best guess so far has been this:
    Cos(220)*5=-cos(40)*5 (I have not been able to simplify this more)
    Sin(220)*5=-sin(40)*5
    and written it as this:
    -(cos(40)5)i-(sin(40)5)j
    Now i controlled this by checking this:
    \sqrt{((sin(220)5)^2+(cos(220)5)^2} =5
    But the answer provided in the back of the textbook is this:
    -\frac{5\sqrt{3}}{2}i-\frac{5}{2}j
    My question is:
    What am I doing wrong? I am completely stuck.
    You have done nothing wrong.
    The answer -\frac{5\sqrt{3}}{2}i-\frac{5}{2}j goes with 240^o and not 220^o.
    Thanks from mariusg
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