To compute the value of arc hyperbolic tangent at x, follow these steps:
Verify your x is a value between -1 and 1. If it is not, you cannot compute tanh inv for this x!
Compute 1 + x and 1 - x.
Perform the division (1 + x)/(1 - x).
Find the natural logarithm of the result.
Divide by 2. That's it!
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