Solution
= Solution
For $\mu\ne0$, the <adjugate matrix> formula, or elimination on the three unit right-hand sides, gives
$$
\boxed{M^{-1}=\begin{pmatrix}\frac1{2\mu}&\frac12&-\frac1{2\mu}\\\frac12&-\frac\mu2&\frac12\\-\frac1{2\mu}&\frac12&\frac1{2\mu}\end{pmatrix}.}
$$
Multiplication by $M$ gives the <identity matrix>, checking all signs and coefficients.