tanh function


Martin McBride, 2020-09-10
Tags tanh
Categories hyperbolic functions pure mathematics

The tanh function is a hyperbolic function. It is also known as the hyperbolic tangent function.

Equation and graph

The tanh function is defined as:

$$ \tanh{x} = \frac{\sinh{x}}{\cosh{x}} $$

Here is a graph of the function:

An alternative equation is:

$$ \tanh{x} = \frac{e^{x}-e^{-x}}{2} \times \frac{2}{e^{x}+e^{-x}} = \frac{e^{x}-e^{-x}}{e^{x}+e^{-x}} $$

tanh as sinh divided by cosh

The tanh function involves two functions, $\sinh{x}$ divided by $\cosh{x}$. This animation illustrates this:

When $x$ is zero, $\sinh x$ is 0 and $\cosh x$ is 1, so the result is 0.

As $x$ gets larger, as the animation shows, $\sinh x$ and $\cosh x$ become ever closer is value, although $\sinh x$ is always slightly smaller. The value of $\tanh x$ therefore tends towards 1.

When $x$ is large and negative, $\sinh x$ becomes ever closer to $-\cosh x$. The value of $\tanh x$ therefore tends towards -1.

Other forms of the equation

If we take the equation:

$$ \tanh{x} = \frac{e^{x}-e^{-x}}{e^{x}+e^{-x}} $$

and multiply top and bottom by by $e^{x}$ we get:

$$ \tanh{x} = \frac{e^{2x}-1}{e^{2x}+1} $$

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