Precalc+trig

Solve tan(40/90) for the double angle cos(2theta). I know I use the x^2 + y^2 = r^2 formula and cos(2theta) = cos^2theta - sin^2theta. But I don't understand my answer

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  1. tan(40/90) ?

    That's an odd notation. Is there some reason why you did not say 4/9? Evan that seems unusual for such a problem.

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