Definition. Let $V$, $W$ be $F$-vector spaces and let $T: V \rightarrow W$ be a function. $T$ is said to be additive if $T(x + y) = T(x) + T(y)$ for all $x, y$ in $V$.
Let $V$, $W$ be $\mathbb{Q}$-vector spaces and let $T: V \rightarrow W$ be an additive function. Prove that $T$ is a linear transformation.