Simplifying gave
The vertices of the box lie on the sphere, so each corner satisfies the equation
[ 4xR^2 - 3x^3 = 0 \quad\Longrightarrow\quad x\bigl(4R^2 - 3x^2\bigr) = 0. ]
A ripple of impressed murmurs ran through the class. The professor nodded, his eyes twinkling. “Excellent,” he said. “You’ve illustrated perfectly how a multivariable problem can sometimes be reduced to one variable, and how the critical point tells us the shape of the optimal object. Well done, Maya.” Simplifying gave The vertices of the box lie
[ y = 2\sqrt{R^2 - \frac{1}{2}\Bigl(\frac{2R}{\sqrt{3}}\Bigr)^2} = 2\sqrt{R^2 - \frac{1}{2}\cdot\frac{4R^2}{3}} = 2\sqrt{R^2 - \frac{2R^2}{3}} = 2\sqrt{\frac{R^2}{3}} = \frac{2R}{\sqrt{3}}. ]
On the central table lay a battered copy of Thomas’ Calculus, 9th edition , its corners softened by years of eager thumbs. A thin, yellowed sheet was tucked between pages 178 and 180, its header scrawled in a hurried hand: . Maya’s professor had hinted that the problem was a “real gem” and that the solution would be discussed the next week—if anyone could actually work it out.
[ \left(\frac{x}{2}\right)^2+\left(\frac{x}{2}\right)^2+\left(\frac{y}{2}\right)^2 = R^2 . ] “Excellent,” he said
which simplified to
[ V'(x) = 4x\sqrt{R^2 - \tfrac{x^2}{2}} - \frac{x^3}{\sqrt{R^2 - \tfrac{x^2}{2}}}. ]
Using the product rule and the chain rule, she obtained ] On the central table lay a battered
Maya wrote the result in bold, underlined it, and added a small smiley face next to it—her personal signature of triumph. The next morning, the professor walked into the seminar room, a stack of papers in his hand. He asked the class to volunteer a solution for Exercise 179. Maya’s hand rose, heart thudding like a metronome.
[ V'(x) = 4x\bigl(R^2 - \tfrac{x^2}{2}\bigr)^{1/2} + 2x^2\left(\tfrac{1}{2}\right)\bigl(R^2 - \tfrac{x^2}{2}\bigr)^{-1/2}(-x) ]
[ \frac{x^2}{2} + \frac{y^2}{4} = R^2. ]
As she walked home, she imagined the inscribed cube—edges perfectly aligned, each corner just touching the sphere—sitting like a gem inside a glass sphere, a concrete reminder that sometimes, the most beautiful solutions are the simplest, and that every calculus problem hides a story waiting to be told.
