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I Confusion in coordinates

  1. Jun 14, 2017 #1
    In the attached pic,it is shown that the coordinates of point B are (a cos theta, a sin theta) ,but shouldn't it be (-a cos theta,a sin theta)? Can anybody please help?

    Attached Files:

  2. jcsd
  3. Jun 14, 2017 #2
    No there is nothing wrong with the labeling of the coordinates in the diagram. Recall that ##\cos \theta## is negative for ##\pi/2 < \theta < 3\pi/2## (or what you may know as the second and third quadrants).
  4. Jun 14, 2017 #3
    But didn't it come like this?(see the attachment)

    Attached Files:

  5. Jun 14, 2017 #4
    When you did the triangle construction in your diagram, you treated ##x## there as a length, which only takes on positive values, but ignored its position relative to where the origin was defined. So, the x-coordinate of the point should in fact be the negative of the ##x## in your derivation.
  6. Jun 14, 2017 #5
    Oh,now I got it,thank you so much:smile:
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