*Stephanie Matalone*Show bio

Stephanie taught high school science and math and has a Master's Degree in Secondary Education.

Lesson Transcript

Instructor:
*Stephanie Matalone*
Show bio

Stephanie taught high school science and math and has a Master's Degree in Secondary Education.

Explore the world of geometry, a mathematical field that studies space, dimensions, and shapes. Learn about area, surface area, and volume. Discover the formulas to calculate the measurements of two-dimensional and three-dimensional shapes.
Updated: 11/27/2021

In its simplest form, **geometry** is the mathematical study of shapes and space. Geometry can deal with flat, two-dimensional shapes, such as squares and circles, or three-dimensional shapes with depth, such as cubes and spheres.

Before diving into two-dimensional and three-dimensional shapes, consider the basic geometric objects that create these shapes: points, lines, line segments, rays, and planes.

A **point** is represented by a dot and shows a location in space. A **line** is a set of straight points that extends forever in both directions as depicted by arrows on both ends. **Rays** are lines that end on one side. **Line segments** end on both sides. **Planes** are surfaces that extend forever in all directions.

**Two-dimensional objects** only have two dimensions: length and width.

**Polygons** are two-dimensional shapes made up of line segments. In order to be considered a polygon, a set of line segments must be closed, meaning each line segment meets up with another line segment. Because of this requirement, squares and triangles are considered polygons, while a circle is not a polygon.

**Squares** are polygons made of four line segments, where each segment is the same length. **Rectangles **are also made of four line segments, where two parallel segments are equal length, and the other two parallel segments are equal length. **Triangles** are polygons with three line segments that can be equal length, but don't need to be.

**Perimeter** is a commonly calculated measurement with two-dimensional shapes in geometry that adds the length of a polygon's line segments. Perimeter calculations are for different applications, including finding how much fencing to put up around your yard.

As illustrated, perimeter calculations are essentially the same for the different shapes: the length of each separate line segment must be added together. For example, if one side of a square, *a*, measures 12 inches, then using the formula for a square's perimeter, 4*a*, *p* is 4 times 12, which equals 48 inches.

The formulas for finding the perimeter of a rectangle and a triangle also require finding the sum of the length of the sides:

Perimeter of a rectangle = (2 * length) + (2 * width)

The perimeter of a triangle = a + b + c

The perimeter measurement of a **circle** is known as **circumference**. The circumference of a circle is found by using **diameter** *d* (the distance across a circle), or **radius** *r* (the distance halfway across a circle), and **pi**, which is a ratio used in geometry that roughly equals 3.14.

Here's an example of how to calculate a circle's circumference. To find the circumference of a circle with a radius *r* of 4 meters, simply multiply 4 by 2 and by pi (3.14). The circumference of that circle would be approximately 25.12 meters. The circumference = 4 * 2 * 3.14

**Area** is the measure of the surface of an object. The area of a square, rectangle, triangle, and circle can be found using formulas. Area calculations are used, for instance, when people want to know the square footage of their homes.

The formula for the area of a square is A = l^2 (l = length of a side). The formula for finding the area of a rectangle is A = length * width. When finding the area of a triangle, the formula area = ½ base * height.

As an example, to find the area of a triangle with a base *b* measuring 2 cm and a height *h* of 9 cm, multiply 1/2 by 2 and 9 to get an area of 9 cm squared. Area is ½ * 2 * 9 = 9.

The formula for the area of a circle A = pi * r^2. This means use 3.14 (for pi) times the radius-squared.

Unlike two-dimensional objects, **three-dimensional** objects have a third dimension, depth, and are, therefore, not flat. Cubes, spheres, and pyramids are examples of three-dimensional objects. A **cube** is an object made of six square sides. A **sphere** is an object shaped like a ball, where every point on the surface is the same distance from the center of the ball. A **cylinder** is another three-dimensional object like a can with two circular ends and curved sides.

When working with three-dimensional objects, formulas are used to find surface area and volume. **Surface area** is similar to perimeter but instead of adding up the length of the line segments, the areas of each of the shapes composing the three-dimensional object are added together. Knowing this, the formulas for these three-dimensional shapes can be derived. For instance, a cube's surface area is 6 times the area of a single square because it's made of 6 squares.

Surface area can be useful in real life when determining how much paint you need to cover an object. Review the formulas for the surface area of the different shapes:

The formula for the surface area of a cube or a rectangular prism is:

SA = 2lw + 2hw + 2lh. And the formula to use with a cylinder is SA = 2B + Ch (B= Area of the base, C = the circumference). To find the surface area of a sphere, SA = 4 * (pr * r^2).

As an example, to find the surface area of a sphere with a radius of 3 feet, simply square the radius and multiply by 4 and by 3.14. The surface area is 113.04 feet squared.

**Volume** is the amount of a space an object takes up. For a cube, this means finding the area of one square, and finding how much stuff can fit inside if this square is stacked the same number of times as the length (or width). So, when solving for a cube's volume, the length of the side can be multiplied by itself three times because its length, width, and depth are equal.

Volume has many real life uses because it calculates how much an object will hold. For example, you might use the volume of a cylinder to find out how much water your bottle will hold. Even further, you may use the volume of a rectangle to find out how much junk your moving truck can hold when you buy a new house.

The formula to find the volume of a rectangular prism is V = lwh. To find the volume of a sphere, use V = 4/3 * (pi * r^3). Here's an example for finding the volume of a sphere with a radius of a sphere, with a radius of 3 m. Start by cubing the radius to get 27 m squared. Then, multiply 4/3 by pi and 27 to get a final answer of 113.04 m cubed.

And finally, to calculate the volume of a cylinder, use the formula V = Bh. (B = Area of the base)

**Geometry** is a mathematical subject that deals with shapes and space. Formulas can be used to find the perimeter and area of **two-dimensional shapes**, such as **polygons** and **circles**. **Perimeters** measure the length of the outside of a two-dimensional object, while **area** represents the space on the surface of a two-dimensional shape.

In geometry, formulas can also be used to find the **surface area** and **volumes** of **three-dimensional shapes**, like **cubes** and **cylinders**. **Volume** measures the amount of space a three-dimensional object takes up. **Surface area** measures the area of all sides of a three-dimensional object.

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