Sum of Ring Products is Subring of Commutative Ring

Theorem
Let $\left({R, +, \circ}\right)$ be a commutative ring.

Let $\left({S, +, \circ}\right)$ and $\left({T, +, \circ}\right)$ be subrings of $\left({R, +, \circ}\right)$.

Let $S T$ be defined as:
 * $\displaystyle S T = \left\{{\sum_{i \mathop = 1}^n s_i \circ t_i: s_1 \in S, t_i \in T, i \in \left[{1 \,.\,.\ n}\right]}\right\}$

Then $S T$ is a subring of $\left({R, +, \circ}\right)$.

Proof
From Sum of All Ring Products is Additive Subgroup we have that $\left({S T, +}\right)$ is an additive subgroup of $R$.

Let $x_1, x_2 \in S T$.

Then:
 * $\displaystyle x_1 = \sum_{i \mathop = 1}^m s_i \circ t_i, x_2 = \sum_{i \mathop = 1}^n s_j \circ t_j$

for some $s_i, t_i, s_j, t_j, m, n$, etc.

Then:

So $x_1 \circ x_2 \in S T$ and the result follows from the Subring Test.