GRE Sample Test 2 Quantitative Reasoning 2

Plug In for k, and let k = 3. CE is a radius and also half of the square’s diagonal. If k is 3, then CE is 3, and the diagonal is 6. The diagonal of a square is also the hypotenuse of a 45:45:90 triangle. To get the hypotenuse from a side, multiply by $\sqrt{2}$; so, to get a side from the hypotenuse, divide by $\sqrt{2}$. The sides of the square are each $\frac{6}{\sqrt{2}}$.
To find the area, square the side to find
${\left(\frac{6}{\sqrt{2}}\right)}^2$ = $\frac{6^2}{\sqrt{2^2}}$ = $\frac{36}{2}$ = 18.
Plug k = 3 into the answers to find one that yields your target of 18.
Choice (E) yields the target of 18.