Figure 3.8.212(b) shows the paper with a generic crease. The blue and red triangles are congruent because the red triangle came, by the fold, from the region in blue. Consequently,

The green and yellow triangles are similar because they are right triangles in which $\angle \mathrm{\θ}$ and $\angle \mathrm{\ψ}$ are complements. Consequently,

Control-drag $L\left(x\right)\=\dots$
Context Panel: Assign Function

Type $L\left(x\right)$ and press the Enter key.

Context Panel: Optimization≻Minimize (local)

The graph of $L\left(x\right)$ in Figure 3.8.12(c) could be examined for an estimate of the minimum value for $L$, but because of the shallow shape of the curve near the minimum, it would be difficult to obtain an accurate answer.

According to the graph, $x\=4$ is a vertical asymptote that corresponds to the paper being folded in half vertically. If the paper were infinitely tall, then $L$ would indeed be unbounded, but of course, the model does not permit $L$ to be greater than the height of the paper.

Write $L\prime \left(x\right)\=0$ and press the Enter key.

Context Panel: Solve≻Solve
(If $x\=8$, the upper left corner will not be on the right edge of the paper.)