# How do you find the exact value of tan(tan^-1(-4.2))?

The value of given expression is $- 4.2$
If a function $f \left(x\right)$ has its inverse function ${f}^{-} 1 \left(x\right)$, then the value of:
$f \left({f}^{-} 1 \left(a\right)\right)$ or ${f}^{-} 1 \left(f \left(a\right)\right)$ is $a$.
In the given example $a = - 4.2$