A point is , as seen in ry1, the limiting dimensionless case of circles (or any other shape) as the dimension gets smaller and smaller .
A line segment is really the shortest distance between two points.
From two different points passes a line.
From three different points not on a line, pass two lines.
D, E and G are on the same line .
For A there are 6 choices; After A, for B 5 choices; For C 4 ;
For D 3 choices but 2 of them are the same.
So the result should be the sum of natural numbers up to 6, minus 1
The adjacent angles, in this case, form a direct angle .
The angles smaller than 90 degrees are called acute .
The angles greater than 90 degrees are called obtuse .
Obviously rechtangle .
Parallel lines do never intersect (or they intersect at infinity) .
A line can not intersect itself.
Two nonparallel lines do certainly intersect somewhere .
1st linee finds 6 lines to intersect, next finds 5, next 4 ... .
Since no parallel lines no. of intersection points 1+2+3+4+5+6
There may be order of infinities ... .
Be careful with DD' and AA' .
Just to teach . Some people tend to use a notation inheriting from set thory (open, half open and closed sets). They prefer [AA'] , [BB') and (CC').
congruent is equal in size and shape .
similar is equal in shape .