Basic Geometry Concepts/Vocabulary

line segment

a part of a line that has two endpoints

plane

flat surface

ray

a part of a line that starts at an endpoint and extends forever in one direction

parallel lines

lines in the same plane that never intersect

intersecting lines

lines that cross at one point

perpendicular lines

lines that intersect at right angles

angle (∠)

a figure formed by two rays with the same endpoint

vertex

The common endpoint of an angle

Name an angle by its...

vertex

right angle

90 degrees

acute angle

less than 90 degrees

obtuse angle

An angle between 90 and 180 degrees

straight angle

180 degrees

complementary

Two angles whose sum is 90 degrees

supplementary angles

Two angles whose sum is 180 degrees

polygon

A closed plane figure made up of line segments

triangle

A polygon with three sides.

rectangle

A parallelogram with four right angles

square

A parallelogram with four congruent sides and four right angles.

quadrilateral

a polygon with four sides

Parallelogram

A quadrilateral with two pairs of parallel sides

trapezoid

a quadrilateral with one pair of parallel sides

equilateral triangle

A triangle with three congruent sides

scalene triangle

a triangle with no congruent sides

isoceles triangle

a triangle that has (at least) 2 congruent sides

acute triangle

a triangle with three acute angles

right triangle

a triangle with one right angle

obtuse triangle

a triangle with one obtuse angle

perimeter

the distance around a polygon

regular polygon

A polygon with equal angles and equal sides.

Perimeter of a triangle

P = a + b + c

Perimeter of a square

P = 4s

Perimeter of a rectangle

P = 2L + 2w

decagon

a polygon with 10 sides

Parts of a circle

diameter, radius, chord, center

diameter

the distance across a circle through its center

Radius

The distance from the center of a circle to any point on the circle

radius formula

r = d/2 or r = 1/2d

circumference

The distance around a circle

pi (π)

3.14

circumference formula

C = 2πr or C = πd

if the diameter is divisible by 7

use π = 22/7

diameter formula

d = 2r or c / π

Pythagorean Theorem

a²+b²=c²

Hypotenuse

the longest side of a right triangle

area

The number of square units required to cover a surface (square units).

Area of a square

A=s² or A=lw

Area of a rectangle

A=lw

Area of a triangle

A=bh/2

Area of a parallelogram

A=bh

Area of a trapezoid

A=h(b1+b2)/2

Area of a circle

A=πr²

triangular prism

A prism that has triangular bases

triangular pyramid

A pyramid with a triangular base.

square pyramid

a pyramid with a square base

surface area

The total area of the surface of a three-dimensional object.

Surface area formula

2(lw + lh + wh)

Volume

The amount of space an object takes up (cubic units)

Volume formula

length x width x height