Minimal-area rectangle, bounding (covering) an acute triangle Given acute triangle $ABC$ such that $AB$ is the longest side. I need to prove that the rectangle $PQRS$ of minimum area covering the whole of triangle $ABC$
has one side (say $PQ$) coincident with $AB$ and the opposite side $RS$ parallel to $PQ$ and passing through $C$.
(The application I have that needs this proof does not guarantee that $ABC$ is an acute triangle, only that point $C$ lies inside the strip defined by  the two perpendiculars to $AB$ passing through points $A$ and $B$.  But if angle $C$ is obtuse, it is much easier to prove that the minimal bounding rectangle is as stated.)
If the statement is untrue, I'd like to know the conditions for it to be wrong.
This is not as trivial (for a generic acute triangle) as it looks.  I can show that the minimal bounding rectangle contains $A$, $B$ and $C$ on its boundary, since otherwise you could "slide" a side of the purported minimal rectangle inward until it does touch one of the vertices of the triangle and get a smaller bounding rectangle. I believe that the last such "sliding" step also shows that one of the vertices on the triangle must coincide with a vertex of the minimal bounding rectangle.
That is how far I have gotten in the forward direction.  Working backwards, I think if one shows that one side of the minimal bounding rectangle coincides with a side of the triangle, it will not be hard to show that it would have to be the longest side.
Help would be appreciated.

As you mentioned, an area minimizing rectangle must contain the vertices $A$, $B$, and $C$ on its boundary, and furthermore one of the vertices must coincide with one of the rectangle's vertices (otherwise there is a portion of the rectangle that we can "cut off").
Suppose $PQRS$ contains $ABC$, with $P$ coincident with $A$. Suppose none of the sides of $ABC$ intersected any of the sides of $PQRS$ outside of vertices. Then $B$ and $C$ cannot be on the edges of the rectangle containing $P$, so they are on $QR$ and $RS$, and they do not coincide with the other vertices of the rectangle. WLOG $B$ lies on $QR$ and $C$ lies on $RS$. To show that such a rectangle cannot be area minimizing, it suffices to show that the area of triangle $ABC$ is strictly less than half of that of $PQRS$. Let $l = PQ = RS$ and $w = PS = QR$, so that the area of $PQRS$ is $lw$. Let $l' = CS$ and $w' = BQ$, so that $l-l' = CR$ and $w - w' = BR$. Then the area of the rectangle outside of the triangle is the sum of the areas of the triangles $PQB$, $BRC$, and $CSP$, which are $\frac{1}{2}PQ*QB = \frac{1}{2}lw'$, $\frac{1}{2}BR*RC = (w-w')(l-l')$, and $\frac{1}{2}CS*SP = \frac{1}{2}l'w$, respectively. Hence,
\begin{align} \text{Area}(ABC) &= \text{Area}(PQRS) - \text{Area}(PQB) - \text{Area}(BRC)-\text{Area}(CSP) \\
&= lw - \frac{1}{2}lw' - \frac{1}{2}(l-l')(w-w') - \frac{1}{2}l'w \\
&=\frac{1}{2}(2lw - lw' - (lw -lw' -l'w +l'w')-lw')\\
&=\frac{1}{2}(lw -l'w')\\
&<\frac{1}{2}lw = \frac{1}{2}\text{Area}(PQRS)
\end{align}
Here, the fact that $B$ and $C$ do not coincide with the vertices of the rectangle implies that both $w'$ and $l'$ are positive. Hence, if none of the sides intersected outside vertices, then the rectangle does not minimize area. It follows that an area minimizing rectangle must have an edge coinciding with an edge of the triangle, and furthermore the two edges must be exactly coincident, since otherwise we can again "cut off" extraneous parts of the rectangle.
