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The Lie product formula extends from matrices to any finite-dimensional Lie group. In a local exponential chart, smooth multiplication and its differential give $\log (\exp (sX)\exp (sY))=s(X+Y)+O(s^2)$. Taking $s=t/m$ and using the one-parameter subgroup identity shows that the $m$th power is $\exp (t(X+Y)+O(m^{-1}))$, proving the displayed limit. This proof needs only the first-order expansion, rather than convergence of the full Baker--Campbell--Hausdorff formula.