[RESOLVED] question on Constructible polygon

please see this image
could anyone explain how (talking about the first figure for equilateral triangles) it signifies an equilateral triangle??

thank you

The line OB needs to be extended.

Check this pic instead:

ok, but then how would i mark this point D in the semi-circle?

Point D is the mid point of line OB.

aha, thank you

:thumb:

Just as a note, the two small check marks on lines BD and OD denote that they are of equal length.