!! used as default html header if there is none in the selected theme. Cross multiplication

# Cross multiplication --- Introduction ---

We know that the multiplication of matrices is not commutative, that is, if $A$ and $B$ are two square matrices of the same size, the matrix $C=AB-BA$ is not zero in general.
This exercise Cross multiplication therefore presents you two matrices $A$ and $C$, of size .And your goal will be to find a matrix $B$ such that $AB-BA=C$ (or $BA-AB=C$, according to the circonstances).
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