Max the Mathemagician is calling for volunteers. He has a magic wand of length 10 that can be broken anywhere along its length (fractional and decimal lengths are allowed). After the volunteer chooses these breakpoints, Max will multiply the lengths of the resulting pieces. For example, if they break the wand near its midpoint and nowhere else, the resulting product is 5×5, or 25. If the product is the largest possible, they will win a free backstage pass to his next show. (Amazing, right?)
You raise your hand to volunteer, and you and Max briefly make eye contact. As he calls you up to the stage, you know you have this in the bag. What is the maximum product you can achieve?
Extra credit: Zax the Mathemagician (no relation to Max) has the same routine in his show, only the wand has a length of 100. What is the maximum product now?
Product $P$ can be achieved by breaking a magic wand of length $w$ into $x$ equivalent pieces. This yields
$$P = \left(\frac{w}{x}\right)^x$$
In order to maximize $P$, first take the $\ln$ of both sides:
$$\ln{P} = x\ln{\frac{w}{x}} = x\left(\ln{w} - \ln{x}\right)$$
Taking the derivative,
$$\frac{P'}{P} = x\left(0 - \frac{1}{x}\right) + \ln{\frac{w}{x}}$$
$$P' = \left(\ln{\frac{w}{x}} - 1 \right) \cdot P$$
$$ = 0 = \left(\ln{\frac{w}{x}} - 1 \right) \cdot \left(\frac{w}{x}\right)^x$$
This implies:
$$1 = \ln{\frac{w}{x}}$$
$$e = \frac{w}{x}$$
$$x = \frac{w}{e}$$
For a wand of length 10.
$$x = \frac{10}{e} \approx 3.7$$
$\left(\frac{10}{3}\right)^3 \approx 37.0370$ and $\left(\frac{10}{4}\right)^4 = 39.0625$
Therefore, dividing the wand into equal quarters yields a maximum product of
$$39.0625$$
For a wand of length 100.
$$x = \frac{100}{e} \approx 36.8$$
$\left(\frac{100}{36}\right)^{36} \approx 9.40 \cdot 10^{15}$ and $\left(\frac{100}{37}\right)^{37} \approx 9.47 \cdot 10^{15}$
Therefore, dividing the wand into 37 equal parts yields a maximum product of:
$$9,474,061,716,781,832.7$$