feat: add more questions.

src/edyn.1130981640.2125817583/was-istbrbrleftmathbfacdotnablarightmathbfrmathbfa.0.md

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+Was ist<br><br>[$]\left(\mathbf{a}\cdot\nabla\right)\mathbf{r}=\mathbf{a}[/$]

src/edyn.1130981640.2125817583/was-istbrbrleftmathbfacdotnablarightmathbfrmathbfa.1.md

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+[$]\mathbf{a}[/$]

src/edyn.1130981640.2125817583/was-istbrbrnablatimesmathbfr.0.md

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+Was ist<br><br>[$]\nabla\times\mathbf{r}[/$]

src/edyn.1130981640.2125817583/was-istbrbrnablatimesmathbfr.1.md

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+[$]\vec{0}[/$]

src/edyn.1130981640.2125817583/was-istbrbrnablecdotmathbfr.0.md

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+Was ist<br><br>[$]\nable\cdot\mathbf{r}[/$]

src/edyn.1130981640.2125817583/was-istbrbrnablecdotmathbfr.1.md

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+[$]3[/$]