Question 1095006: Find sin2x, cos2x, and tan2x if cos=-2/sqrt 5
Answer by ikleyn(52814)   (Show Source):
You can put this solution on YOUR website! .
The answer to this problem is not UNIQUELY DEFINED.
In order for the answer was UNIQUELY DEFINED, it should be pointed, to which quadrant the angle x does belong.
Had you pointed it from the very beginning, you would got the answer quickly / quicker.
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