Folding a rectangular paper and finding the area of the triangle so formed. Given a rectangular sheet of paper ABCD such that the lengths of AB and AD are respectively 7 and 3 cms.Suppose B' and D' are two points on AB and AD respectively such that if the paper is folded along B'D' then A falls on A' on the side DC. Determine the maximum possible area of the triangle  AB'D'.  
I discovered quite a few basic facts that everybody can, but cannot actually make any progress. Please help.


Rectangle $ABCD$ is drawn above with vertex $A$ at the origin.  Let $DA' = x $ then slope of $AA' = \dfrac{3}{x} $ and therefore, slope of $B'D' = - \dfrac{x}{3} $ and we have that $B'D'$ passes through $( \dfrac{x}{2}, \dfrac{3}{2} )$, therefore, its equation is
$y' = \dfrac{3}{2} - \dfrac{x}{3} (x' - \dfrac{x}{2} ) $
$x'$ intercept is  $\dfrac{x}{2} + \dfrac{9}{2x}$ and $y'$ intercept is $\dfrac{3}{2} + \dfrac{x^2}{6} $
We must have the $x'$ intercept in $[0, 7]$, i.e.
$ 0 \le x + \dfrac{9}{x} \le 14 $
So that
$ 0 \le x^2 + 9 \le 14 x $
And finaly $ x^2 - 14 x + 9 \le 0 $, which implies that $x \gt 0.67544 $
And we also have the condition of the $y'$ intercept, namely,
$ AD' = \dfrac{3}{2} + \dfrac{x^2}{6} \le 3 $
From which, we must have $ x \le 3 $
Area of $\triangle AB'D' = [AB'D'] = \dfrac{1}{2} \left(\dfrac{x}{2} + \dfrac{9}{2x} \right) \left(\dfrac{3}{2} + \dfrac{x^2}{6} \right) $
And this simplifies to
$ [AB'D'] = S(x) =  \dfrac{1}{2} \left( \dfrac{3}{2} x + \dfrac{1}{12} x^3 + \dfrac{27}{4x} \right) = \dfrac{1}{8} \left( 6 x + \dfrac{x^3}{3} + \dfrac{27}{x} \right)$
Differentiating this expression with respect to $x$ and equating to zero, gives us,
$ 6 + x^2 - 27/x^2 = 0 $
Multiplying through by $x^2$,
$ x^4 + 6 x^2 - 27 = 0 $
$\Rightarrow (x^2 + 9)(x^2 - 3) = 0 $
And therefore, $x = \sqrt{3} $ gives the critical value for the area.  To identify it as a local maximum or local minimum, we need to check the sign of second derivative.
$S''(x) = \dfrac{1}{8} ( 2 x + \dfrac{54}{x^3} )  \gt 0 $
So, at  $ x = \sqrt{3} $ we have a local minimum.
In addition, we have to check the value of $S(x)$ at the end points $x = 0.67544 $ and $x = 3 $.  We have
$S(0.67544) =   5.5161$
$S (\sqrt{3}) = 3.4641$
$S(3) = 4.5$
Therefore, $x = \sqrt{3} $ corresponds to the minimum area of $\triangle AB'D' $ where it is defined.
