# Two-Dimensional (plane)

## Triangle: Area

The area of a shape is the size of the area inside the boundaries of that two-dimensional shape (once you start working with three dimensional shapes, area will refer to the two-dimensional area of one of the faces of that shape).

To find the area of a triangle, you multiply the height times the length of the base, and then divide that answer by 2.

## Rectangles: Area

The area of a shape is the size of the area inside the boundaries of that two-dimensional shape (once you start working with three dimensional shapes, area will refer to the two-dimensional area of one of the faces of that shape).

Finding the area of a rectangle simply requires you to find the two dimensions of the rectangle (length and width) and multiply them together:

## Perimeter

Perimeter is the measure around the outside of a two-dimensional object. Sometimes it’s best to draw pictures to solve perimeter problems. When you imagine "measuring" perimeter, run your finger all the way around the outside edge of the object. Make sure to include the length of every side that your fingers runs over when you add up the sides (and do not include any interior lines that your finger did not run over!).

## Identifying Quadrilaterals

**Quadrilaterals are four-sided, two-dimensional figures.** We classify quadrilaterals based upon:

## Identifying Shapes (basic)

There are several stages to identifying shapes (and classifications can often become more and more specific).

The first set of shapes that students typically learn are two-dimensional or plane shapes (they have only two dimensions: length and width and occupy a plane).

Plane shapes are made up of lines and vertices (a vertex is a corner, where more than one line meet). Polygon is the general name for a "many sided" shape.

## Geometry Vocabulary and Notation

Geometry is made up of lines, lines segments, rays, angles, polygons, and circles. It’s important to know what these basic geometry units are and how to name them, so that you understand geometry lessons, figures, and notation. Often, the key to solving a geometry problems lies in understanding what the symbols mean.