Proposition 2

Let $a = (a_i)_{i \in I}$, be a family of elements of $A$ and let $u \in A[X]$. If $v$ is the polynomial obtained by substituting $X_i + a_i$ for $X_i$ for each $i \in I$, then the constant term of $v$ is equal to $u(a)$.

The constant term of $v$ is obtained by substituting 0 for $X_i$ in $v$ for each $i \in I$. The result therefore follows by (2).