Let's do a little background first. First of all we know that 
an equilateral triangle with each of its sides equal to 1 has 
three internal 60° angles:

 

Then when we split it down the middle into two congruent
right triangles, like this:



that we split the bottom side in half, so that each part is . 
Then we erase the left half:



Then we can find the vertical side with the Pythagorean
theorem like this:


 Let "a" represnt the side that is  and "c" 
represent the hypotenuse which is , the b
will represent the unknown vertical side:











Now notice that that triangle will fit nicely on the graph,
with its vertex at the given point (,) 



The angle represented by the green arc is 60°. And so obviously,
the angle represented by the red arc must be 180°-60° or 120°.

Edwin