2018 AMC 8 Problem 24

The quadrilateral $EJCI$ is a rhombus, so use its diagonals.

Express the diagonals $IJ$ and $CE$ in terms of the cube side length.

Allow $s$ to represent the length of an edge of the cube. Noting that each side of the cross section is equal in length, we conclude that $EJCI$ is a rhombus. The area of this rhombus can be calculated as $\frac12 IJ\cdot CE,$ as the area of a rhombus is equal to half the product of its diagonals. Using the Pythagorean Theorem: $$IJ=FH=s\sqrt{2}.$$ Similarly, using the Pythagorean Theorem again lets us see that: $$\begin{align*}CE&=\sqrt{AC^2+AE^2}\\&=\sqrt{(s\sqrt{2})^2+s^2}\\&=\sqrt{2s^2+s^2}\\&=s\sqrt{3}\end{align*}$$ Therefore, $$\begin{align*}R&=\dfrac{\frac12 IJ\cdot CE}{s^2}\\&=\dfrac{\frac12 s^2\sqrt{2}\sqrt{3}}{s^2}\\&=\sqrt{\dfrac32}\end{align*}$$ Thus, $R^2=\dfrac32,$ and the correct answer is C.