Answer
Not defined
Work Step by Step
Suppose $\theta =\cos^{-1} (3 \pi)$
This gives: $\cos \theta=\cos[\cos^{-1} (3 \pi)]$
The inverse property states that $\cos(\cos^{-1} a)=a $ satisfies for every $ a $ in the range $[0,\pi]$ .
But the value $ a=3 \pi $ does not lie in the range of $[-1,1]$.
Therefore, we have $\cos[\cos^{-1} (3 \pi)]$= Not defined
