Geometry
Basic concepts:
• Basic concepts aren’t defined, however, they can be described.
• Geometry has three basic concepts: point, line and plane.
Point, line, plane:
• A point has no size. It is represented with a dot and named with capital letters.
• A line has length but neither width not thickness. It may be straight, curved or broken. It extends infinitely far in both directions. Lines can be named with small letters. If A and B are two points on a line, the line can also be named AB (line AB).
• A flat surface may be called a plane. A plane extends infinitely far in all directions and has no thickness. Planes can be represented with four-sided figures and are named with capital letters.
Collinear, non-collinear points:
• Def: collinear points are points which are contained in one line.
• Def: those points which aren’t contained in one line are said to be non-collinear.
Coplanar points or lines:
• Def: coplanar points (or lines) are points (or lines) which are contained in one plane.
• Def: points of lines which are not contained in the same plane are non-coplanar.
Lines:
• Def: if two straight lines intersect, then the intersection is exactly one point.
• Def: two straight lines are said to be parallel if they lie in the same plane and do not have a common point or all points are in common.
• Def: two straight lines are perpendicular to each other when the angles at which they intersect are all equal.