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[Phys-L] area of the shaded triangle



The following puzzle is somewhat amusing.
It went viral recently:
https://amp.businessinsider.com/images/5ae3808042e1cc5185248f88-750-563.jpg

It might be useful as a start-of-term pretest, to see
if students actually learned anything in HS math.

Note: The steeply sloping line meets the *midpoint* of
the top edge. I reckon the tick marks in the diagram
are supposed to indicate this, but it's a better puzzle
if solving it does not require knowing this obscure bit
of notation.

===============

Would anybody care to comment on the following criteria
for judging answers:

-- Some traction (as opposed to just giving up).
-- Numerical solution, but not correct.
-- Correct numerical solution.
-- Simple, elegant, insightful solution.

I would argue that the above leaves out three things,
all somewhat related. Hint/giveaway:

Fgneg ol bognvavat hccre naq ybjre obhaqf.
Purpx gur jbex hfvat obhaqf be bgurejvfr.
N erny zngurzngvpvna jbhyq sbezhyngr gura
fbyir gur trarenyvmrq ceboyrz, abg whfg
gur zvqcbvag pnfr.

Meta-hint: http://www.rot13.com/