Leading Coefficient of Product of Polynomials over Integral Domain

Theorem
Let $R$ be an integral domain.

Let $f, g \in R[x]$ be polynomials.

Let $c$ and $d$ be their leading coefficients.

Then $fg$ has leading coefficient $cd$.

Proof
Let $p = \deg f$ and $q = \deg g$ be their degrees.

By Degree of Product of Polynomials over Integral Domain, $\deg(fg) = p + q$.

For a natural number $n \geq 0$, let:
 * $a_n$ be the coefficient of the monomial $x^n$ in $f$.
 * $b_n$ be the coefficient of the monomial $x^n$ in $g$.

By Coefficients of Product of Two Polynomials, the coefficient of $x^{p+q}$ in $fg$ is the sum:
 * $\displaystyle \sum_{k \mathop = p}^p a_k b_{p+q-k}$

By Indexed Summation over Interval of Length One:
 * $\displaystyle \sum_{k \mathop = p}^p a_k b_{p+q-k} = a_p b_q = cd$