How can I tell that is a 30/60 right triangle?.........the def feature is the 90 degree angle........0 - 9........all 10 single digit numbers..................11 + 13 + 17 + 19 = 60.....................those 4 end digits.......1, 3, 7, 9...............4 end digits.....any prime > 10 can end..............40% of the number line......> 10..............60 % was cut away..........30/60 degree right triangle.........1/2..............30/60 = 1/2...............

The hyp is one............b/c the radius of the unit circle is one..................the sine of 30 degrees is 1/2...........which is the distance from origin.....................where the two perpendicular black lines cross......................to the base of the horizontal blue line;.....is 1/2...........I know that from the other lines...........which I did not draw here.................the sine of 60 degrees is 1/2 the square root of 3................it matches............b/c u KNOW the radius is 1................