# Points, Lines, Angles, and Planes

### Terms

### Acute

An adorable little angle. By "little," of course we mean "smaller than 90 degrees in measure."### Adjacent

The conjoined twins of the geometry world. Adjacent angles always share a vertex and a side.### Angle

The corner of empty space between two lines, rays, or segments that share a point. Usually, angles are measured in degrees (and most of them have at least a bachelor's degree).### Bisected

Split in two. We can say that an angle is bisected, a segment is bisected, or that the Dakotas are bisected.### Bisector

Something that splits another thing in half. Like a ray being an angle bisector, or a point bisecting a line segment, or a chainsaw bisecting a zombie.### Collinear

Used to describe points that are all on the same line.### Complementary Angles

Two wrongs don't make a right, but two complementary angles do. They're two angles that add up to 90° exactly.### Congruent

### Coplanar

Used to describe lines or points that are all on the same plane.### Distance Formula

A formula that calculates distances based on the coordinates we plug into it. It takes the form of this lovely thing: .### Length

The distance between two endpoints. Can be expressed in units of millimeters, inches, feet, miles, or light-years—they're all lengths.### Line

A one-dimensional segment that continues on forever in both directions. Time-consuming to draw, so we use arrows on the ends to symbolize that it never ends.### Line Segment

Sometimes just called a "segment." It's a finite piece of line between two endpoints.### Midpoint

A point that bisects a line segment into two congruent segments. It's the halfway point.### Obtuse

An angle larger than 90° in measure. It has nothing to do with their smarts.### Origin

The point (0, 0) on the*x*-

*y*graph. In 3D space, it's (0, 0, 0). For any additional dimensions, just add more zeros.

### Parallel

Two lines that are on the same plane but never intersect. They're always in sight, but never touch…sort of sad, ain't it?### Perpendicular

Two lines that create a 90° angle when they intersect. Well, actually they create four 90° angles, but who's counting?### Plane

A "slice" of three-dimensional space. It has length and width, but no depth, like a sheet of paper that stretches out forever in all directions.### Point

The smallest object…ever. It has no mass, no length, and no size. It describes only a location.### Ray

A hybrid of a line and a segment. It has one endpoint, but then goes off forever in the other direction. It's like a ray of sunshine that starts at the sun and then continues on forever.### Right Angle

An angle that's exactly 90°. Naturally, that means any angle that isn't 90° is wrong.### Straight Angle

An angle that measures 180°. The title a straight line gives itself when it wants to seem more impressive.### Supplementary Angles

Two angles that add up to 180°. It doesn't matter if they're adjacent or on different planets.### Vertical Angles

Angles that are opposite each other resulting from two intersecting lines. Vertical angles are*always*congruent.

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