How do I find my angle using sin and cos values?

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Discussion Overview

The discussion revolves around the process of determining an angle from given sine and cosine values. It includes theoretical aspects of trigonometric functions and their inverses, as well as practical methods for visualizing angles on a unit circle.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about retrieving an angle from known sine and cosine values.
  • Another participant suggests using inverse sine and cosine functions to find the angle.
  • A participant expresses confusion regarding the output of the inverse cosine function, indicating that it does not yield the expected angle.
  • Further explanation is provided, proposing two methods: visualizing graphs of sine and cosine functions or using a unit circle to determine the angle based on the signs of sine and cosine values.
  • The method involving the unit circle is elaborated, explaining how to identify the quadrant of the angle based on the signs of sine and cosine, and how to adjust the angle accordingly.
  • A later reply confirms the effectiveness of the suggested method.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach, as there are differing views on how to interpret the results of inverse trigonometric functions and the methods for visualizing angles.

Contextual Notes

The discussion highlights potential limitations in understanding the outputs of inverse functions and the importance of quadrant considerations when determining angles from sine and cosine values.

Ascending One
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So, I have the values for sin (angle) and cos (angle)... how do I get back to my angle?
 
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Use the inverse sin and cos functions.
 
But say my angle is 200. So I have inverse cosine(cosine (200)) which equals... 160. That's not what I want.
 
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.
 
I used your method. Thank you!
 

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