Surface Area of 3-D Figures - Pyramids Formulas | List of Surface Areas of 3-D Figures - Pyramids Formulas You Should Know - BYJUS

# Surface Areas of 3-D Figures - Pyramids Formulas

Pyramids are solid shapes obtained by joining the vertices of a polygonal base to a point outside its plane, known as the apex. We can find the area of the pyramid by finding the sum of the individual areas of the two-dimensional faces, or by using a formula derived from the same concept. ...Read MoreRead Less

### Determining the Surface Area of Solid Shapes

The area of a shape is the extent of the region covered by the shape on a plane. The surface area of a three-dimensional shape refers to the total amount of space occupied by all of its faces. We can use the concept of nets to find the surface area of three-dimensional solids. A net is an arrangement of two-dimensional shapes that can be folded to obtain a three-dimensional shape. Basically, the net of a shape is the same as what a 3D shape would look like if it was opened and laid out.

### List of Formulas

We can find the area of a pyramid by adding the area of its base and the lateral surface area. As the lateral surfaces are congruent triangles, we can use a formula to find the lateral surface area in one go.

• The surface area of a pyramid = Area of base + Sum of areas of lateral surfaces
• The surface area of a regular pyramid = $$\frac{1}{2}Pl+B$$

Here, $$P$$ is the perimeter of the base, $$l$$ is the height of the triangular faces, which is also the slant height of the pyramid, and $$B$$ is the area of the base.

The first equation is similar to the second equation. In the first equation, we find the base area and the areas of the individual lateral surfaces. Then we add these areas together to get the surface area.

In the second equation, we are calculating the total area of the lateral surfaces and the area of the base. We can use this idea as the triangular faces in a regular pyramid are congruent. Instead of using the $$\frac{1}{2}\times b \times h$$ equation for each triangle, we find the sum of the bases of the triangles — which is the same as the perimeter of the base — and use the equation $$\frac{1}{2}\times P \times l$$ to find the total area of the lateral surfaces. Here, P is the perimeter of the base, and l is the height of the triangle, which is also the slant height of the pyramid.

Alternatively, we can use the nets of the pyramids to find the total area of the pyramid by calculating the sum of the areas of the individual faces.

### Solved Examples

Example 1: Find the total area of the pyramid.

Solution

The length of the edges of the base of the pyramid is 5 inches. Since the pyramid has a square base, it is a regular pyramid. The slant height of the pyramid is 8 inches.

The surface area of a regular pyramid = $$\frac{1}{2}Pl+B$$

The perimeter of the base, P = 5 + 5 + 5 + 5 = 20 inches

The slant height of the triangular face, $$l=8$$ inches

The lateral surface area = $$\frac{1}{2}Pl$$

= $$\frac{1}{2}\times 20 \times 8$$

= 80 square inches

Base area, B = $$a^2$$

= $$5^2$$

= 25 square inches

So, the total surface area of the pyramid = $$\frac{1}{2}Pl+B$$

= 80 + 25

= 105 square inches

Example 2: Find the surface area of a square pyramid whose base length is 16 inches and the slant height is 17 inches

Solution

Since it is a square pyramid, we can use the formula used to find the surface area of a regular pyramid.

The surface area of a regular pyramid = $$\frac{1}{2}Pl+B$$

Base perimeter, P = 16 + 16 + 16 + 16

= 64 inches

The area of the base, B = $$a^2$$

= $$16^2$$

= 256 square inches

So, the surface area of a regular pyramid = $$\frac{1}{2}Pl+B$$

= $$\frac{1}{2}\times 64\times 17+256$$

= $$32\times 17+256$$

= 544 + 256

= 800 square inches

Hence, the surface area of the pyramid is 800 square inches.

Example 3: A structure in the shape of a triangular pyramid is placed on a flat surface which needs to be covered with a piece of cloth. The base of the triangular base is 3 feet long and the slant height of the pyramid is 5 feet. Find the area of the cloth used to cover the pyramid.

Solution:

To find the area of the cloth used to cover the pyramid, we need to find the area of the lateral surfaces of the pyramid, as the base of the pyramid is not covered with cloth.

Base length of the triangular base of the pyramid = 3 feet

The slant height of the pyramid = 5 feet

Area of one triangular face = $$\frac{1}{2}\times b \times h$$

= $$\frac{1}{2}\times 3 \times 5$$

= 32.5

= 7.5 square feet

Since this is a triangular pyramid, there are three lateral surfaces.

So, lateral surface area = 7.53

= 22.5 square feet

So, the area of the cloth used to cover the structure is 22.5 square feet.

Example 4: Find the surface area of the regular pyramid below.

Solution:

The length of the base edge of the pyramid is 6 inches, and the distance between the apex of the pyramid and the vertex is 5 inches.

Since this is a regular pyramid we will use the formula to find the surface area of a regular pyramid.

The surface area of a regular pyramid = $$\frac{1}{2}Pl+B$$

The perimeter of the base, P = 6 + 6 + 6 + 6 = 24 inches

Area of the base, B = $$6^2=36$$ square inches

In order to determine the surface area of the pyramid, we need to find the slant height of the pyramid using the Pythagorean Theorem.

To use the Pythagorean Theorem, take a look at one of the faces of the pyramid. The slant height is perpendicular to the edge of the base.

Hence, $$a^2+b^2=c^2$$

Here, $$b=\frac{6}{2}=3$$ inches

$$c=5$$ inches

$$a^2+3^2=5^2$$

$$a^2=25-9$$

$$a^2=16$$

$$a=\sqrt{16}$$

$$a=4$$ inches

So, the slant height ($$l$$) of the triangular faces is 4 inches.

Therefore, the surface area of a regular pyramid = $$\frac{1}{2}Pl+B$$

= $$\frac{1}{2}\times 24 \times 4+36$$

= 48 + 64

= 112 square inches

So, the surface area of the pyramid is 112 square inches.