(Degree Rule) Let $D$ be an integral domain and $f(x), g(x) \in D[x]$. Prove that deg $(f(x) \cdot g(x))=\operatorname{deg} f(x)+\operatorname{deg} g(x) .$ Show, by example, that for commutative ring $R$ it is possible that $\operatorname{deg} f(x) g(x)<$ $\operatorname{deg} f(x)+\operatorname{deg} g(x)$, where $f(x)$ and $g(x)$ are nonzero elements in $R[x]$. (This exercise is referred to in this chapter, Chapter 17 , and Chapter 18.)