Let $$
\mathbf{r}_{1}(t)=\left\langle t^{2}, 1,2 t\right\rangle, \quad \mathbf{r}_{2}(t)=\left\langle 1,2, e^{t}\right\rangle
$$
Compute $\frac{d}{d t} \mathbf{r}_{1}(t) \cdot \mathbf{r}_{2}\left.(t)\right|_{t=1}$ in two ways: (a) Calculate $\mathbf{r}_{1}(t) \cdot \mathbf{r}_{2}(t)$ and differentiate. (b) Use the Product Rule.