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College Trigonometry, 2011a

College Trigonometry, 2011a

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826 Foundations of <strong>Trigonometry</strong><br />

To find where this equivalence is valid we check back with our substitution t =arctan(x).<br />

Since the domain of arctan(x) is all real numbers, the only exclusions come from the<br />

values of t we discarded earlier, t = ± π 4 . Since x = tan(t), this means we exclude<br />

x =tan ( ± π )<br />

4 = ±1. Hence, the equivalence tan(2 arctan(x)) =<br />

2x<br />

holds for all x in<br />

1−x 2<br />

(−∞, −1) ∪ (−1, 1) ∪ (1, ∞).<br />

(b) To get started, we let t = arccot(2x) sothatcot(t) =2x where 0

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