MHB How Do You Prove Equidistance in This Isosceles Right Triangle Geometry Problem?

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    2017
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The problem involves an isosceles right triangle ABC with points D and E on sides AC and BC, respectively, such that CD equals CE. The task is to prove that the lengths of segments LK and LB are equal, where L and K are the intersection points of perpendiculars from C and D to line AE with the hypotenuse AB. A solution has been provided by a user named Opalg, confirming the proof of equidistance. The discussion emphasizes the geometric relationships and properties inherent in isosceles right triangles. Understanding these concepts is crucial for solving similar geometry problems.
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Here is this week's POTW:

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On the sides $AC$ and $BC$ of an isosceles right-angled triangle $ABC$, points $D$ and $E$ are chosen such that $|CD|=|CE|$. The perpendiculars from $C$ and $D$ on $AE$ intersect the hypotenuse $AB$ at $L$ and $K$ respectively. Prove that $|LK|=|LB|$.

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Congratulations to Opalg for his correct solution, which you can find below::)

[TIKZ][scale=0.5]
\coordinate [label=left: $A$] (A) at (0,10) ;
\coordinate [label=below: $B$] (B) at (10,0) ;
\coordinate [label=below: $C$] (C) at (0,0) ;
\coordinate [label=above left: $D$] (D) at (0,4) ;
\coordinate [label=below: $E$] (E) at (4,0) ;
\coordinate [label=above right: $K$] (K) at (4.29,5.71) ;
\coordinate [label=above right: $L$] (L) at (7.14,2.86) ;
\coordinate [label=below: $P$] (P) at (-10,0) ;
\draw [very thick](C) -- (A) -- (B) -- cycle ;
\draw (B) -- (P) -- (K) ;
\draw (A) -- (E) ;
\draw (C) -- (L) ;
[/TIKZ]

Extend the lines $KD$ and $BC$ so that they meet at $P$. Since $KP$ is perpendicular to $AE$, and $PC$ is perpendicular to $AC$, it follows that the angles $KPC$ and $EAC$ are equal. Thus the right-angled triangles $EAC$ and $DPC$ have the same angles. But $|CE| = |CD|$, so those two triangles are in fact congruent. Therefore $|PC| = |CB|$, so that $C$ is the midpoint of $PB$.

The lines $CL$ and $PK$ are parallel, which means that the triangles $CLB$ and $PKB$ are similar. Since $C$ is the midpoint of $PB$, it follows that $L$ is the midpoint of $KB$. Thus $|LK| = |LB|$.
 
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