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**Kakariki** Okay, I think I understand. As both of the variables were "a" I thought that that would not work, but since one is 2a and the other is a, I am to treat them as different variables?

So I would use the formula: Cos (x + y) = cos x cos y - sin x sin y

Which means: Cos 2a cos a - sin 2a sin a

= cos (2a + a)

= cos (3)

Can someone explain why a and 2a are treated as "apples and oranges"? Because, assuming I am understanding correctly, 2a + a = 3a, so why are they being considered as x and y?

Thanks.