= Solution
Take $u=v=l_\xi$ and $w=l_\eta$ in the identity from part (e). For a <left-invariant metric>, all three scalar products are constant and $[l_\xi,l_\xi]=0$, so
$$
\langle\nabla_{l_\xi}l_\xi,l_\eta\rangle
=-\langle\xi,[\xi,\eta]\rangle.
$$
The curve $\gamma_\xi$ has velocity $l_\xi$, hence is a <geodesic> exactly when $\nabla_{l_\xi}l_\xi=0$. Nondegeneracy of the inner product now gives
$$
\gamma_\xi\text{ is geodesic}
\quad\Longleftrightarrow\quad
\langle\xi,[\xi,\eta]\rangle=0
\quad\text{for every }\eta\in\mathfrak g.
$$
This is the <geodesic-vector criterion for a left-invariant metric>.
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