.
= -1. ANSWER
It is a Table angle (= the angle from the Table).
+----------------------------------------------------------------+
| So the value of sine of this angle can be considered as |
| a PRIMARY information, similar to an axiom of parallel lines. |
+----------------------------------------------------------------+
(Same as the case 2 x 2 = 4 from the multiplication Table).
Double angle identity is IRRELEVANT.
Solved.
-------------
The conclusion
A request in this post is semi-idiotic.
P.S. Above, I used the term "semi-idiotic" to make my statement as " soft " as possible.
Without this softening, the statement is " A request in the post is idiotic ". 100% true.