There is no reason that an arbitrary angle can mean a constructible
angle. It depends on the context you are working in.
For example in the context of natural numbers:
An arbitrary natural number means: n in N
A natural number between 10 and 15 means: n in {10,11,12,13,14,15}
The word arbirtrary is neither antonym nor otherwise in
any relationship to the word constructible.
It only indicates that in the current domain you are working,
you pick any element from the domain.
If your domain is some constructible angles A, then arbitrary
means from this domain A.
The word arbitary also indicates somehow that the placeholder
is an input, and not an output dependent on some input.
Since an output cannot be arbitrary anymore, it depends on
the input. So if we for example have as input:
Input:
n : an arbitrary natural number
And as output the succeessor:
Output:
m = S(n) : The successor of n
The m cannot be arbitrary (in German beliebig) anymore,
if both n and m were arbitrary we could choose:
n m
1 1
2 1
1 2
2 2
etc..
But since m=S(n) we can only have:
n m
1 2
2 3
3 4
etc..
Thats also why you don't understand angle trisection,
or any other math here, in angle trisection:
Input:
a : An arbitrary angle
Output:
b = a/3 : The trisected angle
Angain a and b cannot be both arbitrary anymore, since
this would allow:
a b
360 360
360 120
120 360
120 120
etc..
But for trisection we need:
a b
360 120
etc..
Get some skills Stevie Wonder. Skills is everything.
The word arbitrary is pretty standard in math. Its the
most easy way to say in passing that something is input
in a construction. You find it in dozen texts, here are
some samples:
"It concerns construction of an angle (=output) equal to
one third of a given arbitrary (=input) angle"
https://en.wikipedia.org/wiki/Angle_trisection
"it's suppose to be impossible to trisect (=ouput) an
arbitrary (=input) angle with just a compass and
straight-edge"
https://math.stackexchange.com/a/2181844/4414
The word arbitrary has of course a further connotation,
here, since some angles are indeed trisectible, it highlights
that we are asking for a general solution.
It asks whether trisection works always, so for each
input can we get an output?