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    For alpha I did inverseTAN (-2/-5) and got 0.38 radians says the answer is -2.76 I don't know how?

    (Original post by khanpatel321)

    For alpha I did inverseTAN (-2/-5) and got 0.38 radians says the answer is -2.76 I don't know how?
    If you go back and say that Rsinalpha * sinx = -2 then you will see why 0.38 doesn't work but -2.76 works.

    Notice that -2.76 looks suspiciously like 0.38 - pi (3.1415926...)

    This ambiguity between quadrants is a feature of the arctan function. If, as in this case, you have both the y and x components of the arctan, you should always look at the respective signs to see what quadrant the angle must be in. With the y and x components both negative, the angle must be in the third quadrant. That means that even if your calculator's arctan function returns a result of 0.38, the actual angle must be pi+0.38 or -(pi-0.38). These two forms of the answer are 2pi apart, and are therefore equivalent.
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