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No, actually the method is not correct. Adiya needed to take x2 and subtract it from 20x. she would then get 6 = 18x. Have in mind also that she needed to divide 18. So 6 = 18x turns into 6=X. That is why is incorrect
One way it's useful is in the application of phasors in physics. Phasors require you to add up vectors of two different angles, so the cosine angle addition formula can be used if you already know the cosine and sine of the original vector angles.