how would i find the range of validity of an already expanded binomial??

1 + cos(x) + cos^(2)(x) + … + cos^(n)(x) + … is a geometric series where r = cos(x) and a = 1. Find the bounds for r and solve for x (better to find what r cannot be).

Edit: since trig functions oscillate you would also have to exclude their points of oscillation. (This is why the geometric series doesn’t allow r = -1).

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