MHB How Long is Side CD in a Special Trapezium?

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In the discussion about the trapezium $ABCD$, where $AD$ is parallel to $BC$, the problem involves finding the length of side $CD$ given specific conditions. It is established that $BC = BD = 1$, $AB = AC$, and $CD < 1$, with the angles $\angle BAC$ and $\angle BDC$ summing to 180 degrees. The correct solution was provided by members Olinguito and Opalg. The solution process involves geometric properties and relationships within the trapezium. The final answer for the length of side $CD$ is derived through these considerations.
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Here is this week's POTW:

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$ABCD$ is a trapezium with $AD$ is parallel to $BC$. Given that $BC=BD=1,\, AB=AC$, $CD<1$ and $\angle BAC+\angle BDC=180^\circ$, find $CD$.

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Congratulations to the following members for their correct solution!(Cool)

1. Olinguito
2. Opalg

Solution from Opalg:
[TIKZ]\coordinate [label=below left: $A$] (A) at (5,-4.05) ;
\coordinate [label=above: $B$] (B) at (0,0) ;
\coordinate [label=above: $C$] (C) at (10,0) ;
\coordinate [label=below: $D$] (D) at (9.14,-4.05) ;
\draw (A) -- (B) -- node[above] {$1$} (C) -- (D) -- (A) -- (C) ;
\draw (B) -- node[above] {$1$} (D) ;
\node at (5,-3.7) {$\alpha$};
\node at (9,-3.7) {$\beta$};
\node at (5.5,-3.9) {$\gamma$};
\node at (9.5,-0.2) {$\gamma$};
\node at (9.7,-0.5) {$\gamma$};
\node at (0.7,-0.3) {$\gamma$};[/TIKZ]

Let $\alpha = \angle BAC$ and $\beta = \angle BDC$, so that $\alpha + \beta = 180^\circ$. Also, let $\gamma = \frac12\beta$.

In the isosceles triangle $ABC$, the angle at $A$ is $\alpha$, so the other two angles must be $\frac12(180^\circ - \alpha) = \gamma$. Since $\angle BCD = \beta$, it follows that $\angle ACD = \gamma$.

Since $BC$ and $AD$ are parallel, $\angle CAD = \angle BCA = \gamma$. Thus the triangle $ACD$ is isosceles and therefore $AD = CD$.

From the isosceles triangle $BCD$ it follows that $AD = CD = 2\cos\beta$.

Now take coordinates with $B$ at the origin and $C = (1,0)$. The $x$-coordinate of $A$ is $\frac12$, and the $x$-coordinate of $D$ is $1 - CD\cos\beta = 1 - 2\cos^2\beta$. But $A$ and $D$ have the same $y$-coordinate and so $AD = \frac12 - 2\cos^2\beta$.

Therefore $ \frac12 - 2\cos^2\beta = 2\cos\beta$, from which $4\cos^2\beta + 4\cos\beta - 1 = 0$, a quadratic with solution $\cos\beta = \frac12(\sqrt2 - 1)$ (the other solution is less than $-1$, so cannot be a cosine).

Hence $CD = 2\cos\beta = \sqrt2 - 1$.
 
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