Solution
= Solution
In the <quadratic form>, the two equal off-diagonal contributions together supply the mixed term. Comparing coefficients gives
$$
\boxed{M=I_0\begin{pmatrix}2&-1\\-1&2\end{pmatrix}.}
$$
For a nontrivial physical <moment of inertia> scale take $I_0>0$; then this <inertia tensor> is positive definite.