[spec] start vector calculus

diff --git a/spec/vectors.md b/spec/vectors.md
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--- a/spec/vectors.md
+++ b/spec/vectors.md
@@ -193,3 +193,11 @@ $$\boldsymbol{r}(t)=x\boldsymbol{i}+y\boldsymbol{y}$$
 - If $\boldsymbol{r}(t)$ represents position with time, then the graph of endpoints of $\boldsymbol{r}(t)$ represents the Cartesian path.
 - Domain of $\boldsymbol{r}(t)$ is the range of $x(t)$
 - Range of $\boldsymbol{r}(t)$ is the range of $y(t)$
 - If $\boldsymbol{r}(t)$ represents position with time, then the graph of endpoints of $\boldsymbol{r}(t)$ represents the Cartesian path.
 - Domain of $\boldsymbol{r}(t)$ is the range of $x(t)$
 - Range of $\boldsymbol{r}(t)$ is the range of $y(t)$

+
+## Vector calculus
+
+### Derivative
+
+Let $\boldsymbol{r}(t)=x(t)\boldsymbol{i} + y(t)\boldsymmbol(j)$. If both $x(t)$ and $y(t)$ are differentiable, then:
+
+$$\boldsymbol{r}(t)=x(t)\boldsymbol{i}+y(t)\boldsymbol{j}$$