Inverse Tangent of Imaginary Number/Proof 1
Theorem
- $\map {\tan^{-1} } {i x} = i \tanh^{-1} x$
Proof
| \(\ds y\) | \(=\) | \(\ds \map {\tan^{-1} } {i x}\) | ||||||||||||
| \(\ds \leadsto \ \ \) | \(\ds \tan y\) | \(=\) | \(\ds i x\) | Definition of Inverse Tangent | ||||||||||
| \(\ds \leadsto \ \ \) | \(\ds i \tan y\) | \(=\) | \(\ds - x\) | $i^2 = -1$ | ||||||||||
| \(\ds \leadsto \ \ \) | \(\ds \map \tanh {i y}\) | \(=\) | \(\ds -x\) | Tangent in terms of Hyperbolic Tangent | ||||||||||
| \(\ds \leadsto \ \ \) | \(\ds i y\) | \(=\) | \(\ds \map {\tanh^{-1} } {-x}\) | Definition of Inverse Hyperbolic Tangent | ||||||||||
| \(\ds \leadsto \ \ \) | \(\ds i y\) | \(=\) | \(\ds -\tanh^{-1} x\) | Inverse Hyperbolic Tangent is Odd Function | ||||||||||
| \(\ds \leadsto \ \ \) | \(\ds y\) | \(=\) | \(\ds i \tanh^{-1} x\) | multiplying both sides by $-i$ |
$\blacksquare$