12.16 Split the number 18 into two addends such that the sum of their squares is the smallest possible.
12.16 Let $x$ be the first addend; then $18-x$ is the second addend. According to the condition, we get the function $S(x)=x^{2}+(18-x)^{2}$, where by the context of the problem $00$. Thus, $x=9$ is the minimum point of $S(x)$, and therefore, each addend is equal to 9.

Answer: 9 and 9.