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How do I get my angle?

  1. Mar 3, 2008 #1
    So, I have the values for sin (angle) and cos (angle)... how do I get back to my angle?
     
  2. jcsd
  3. Mar 3, 2008 #2

    Kurdt

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    Use the inverse sin and cos functions.
     
  4. Mar 3, 2008 #3
    But say my angle is 200. So I have inverse cosine(cosine (200)) which equals... 160. That's not what I want.
     
  5. Mar 3, 2008 #4

    HallsofIvy

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    You can do either of two things:
    1) Use, if only in your head, graphs for y= sin(x) and y= cos(x), draw horizontal lines at the value of sin(x) on the first and cos(x) on the second.

    2) (And I think this is what you really want since I suspect you are requiring the angle be between 0 and 360) draw, again if only your head, a "unit circle" (circle on a coordinate system with center at (0,0) and radius 1) and remember that (cos(t),sin(t)) are the coordinates of the point at angle t, counter-clockwise around the circle from the positive x-axis. You can distinguish between the various values by looking at signs.
    For your example, where sin(t)= -.3402 and cos(t)= -.9397, since both are negative, you are in the 3rd quadrant. You know immediately that t is between 180 and 270 degrees. Using "inverse sine", or arcsin, of -.3402 on a calculator (have made sure it is set to "degree mode"!) you get -20 degrees which is in the fourth quadrant. The corresponding angle in the third quadrant (20 degrees below the x-axis just as -20 is) is 180+ 20= 200 degrees. If I had used inverse cosine of -.9397, would have gotten 160 degrees, in the 2nd quadrant, 180- 160= 20 degrees above the x-axis. Knowing the the angle I want is in the 4th quadrant, I know it must be 20 degrees below the x-axis: 180+ 20= 200 degrees, again.
     
  6. Mar 3, 2008 #5
    I used your method. Thank you!
     
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