2006 AMC 12A 第 21 题

For $j = 1, 2,$ rewrite the condition as $j + x^2 + y^2 \le 10^j(x + y)$

Complete the square to get disks, then compare their squared radii

For $j = 1, 2,$ the condition becomes $j + x^2 + y^2 \le 10^j(x + y),$ i.e. $$\begin{gathered} \left(x - \frac{10^j}{2}\right)^2 \\ {}+ \left(y - \frac{10^j}{2}\right)^2 \\ \le \frac{10^{2j}}{2} - j. \end{gathered}$$

These are disks with squared radii $\tfrac{100}{2} - 1 = 49$ for $S_1$ and $\tfrac{10000}{2} - 2 = 4998$ for $S_2.$

The logarithms require $x+y>0.$ For each disk, the squared distance from its center to the line $x+y=0$ is $\tfrac{10^{2j}}{2},$ which is $j$ more than the squared radius. Hence both disks lie entirely in $x+y>0,$ so no points were added when the inequalities were rewritten. The area ratio is $\dfrac{4998}{49} = 102.$

Thus, the correct answer is E.