If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: 6th grade (WNCP)>Unit 3

Lesson 2: Perimeter and Area

# Perimeter & area of composite shapes

Sal finds perimeter and area of a non-standard polygon. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

• I need to find the surface area of a pentagonal prism, but I do not know how. Can someone tell me?
• For any three dimensional figure you can find surface area by adding up the area of each face. A pentagonal prism 7 faces: it has 5 rectangles on the sides and 2 pentagons on the top and bottom.

It is simple to find the area of the 5 rectangles, but the 2 pentagons are a little unusual. I don't know what lenghts you are given, but in general I would try to break up the unusual polygon into triangles (or rectangles). If I am able to draw the triangles so that I know all of the bases and heights, I can find each area and add them all together to find the total area of the polygon.

This method will work here if you are given (or can find) the lengths for each side as well as the length from the midpoint of each side to the center of the pentagon. Without seeing what lengths you are given, I can't be more specific. Depending on the problem, you may need to use the pythagorean theorem and/or angles.
• would finding out the area of the triangle be the same if you looked at it from another side?
• This is a 2D picture, turn it 90 deg. in either direction, you just see a line going up and down, turn it 45 deg. you have the same picture, just narrower, so no. But if it was a 3D object that rotated around the line of symmetry, then yes.
• Why do you use 8 when multiplying it with the 3 to find the area of the triangle part instead of using 4?
• As he says at , for a triangle the formula is base times height times 1/2. The base of this triangle is 8, and the height is 3.
• How did you get the 3 inches?
(1 vote)
• Area of polygon in the pratice it harder than this can someone show way to do it? thank
• It's pretty much the same, you just find the triangles, rectangles and squares in the polygon and find the area of them and add them all up.
• i dnt understand....Why do you use 8 when multiplying it with the 3 to find the area of the triangle part instead of using 4?
• Sal messed up the number and was fixing it to 3. You'll notice the hight of the triangle in the video is 3, so thats where he gets that number.
• what is a perimeter ? and i need it in mathematical words
• Perimeter is defined as the distance around the outside of a geometrical shape.
• What exactly is a polygon?
• A polygon is a closed figure made up of straight lines that do not overlap. If a shape has a curve in it, it is not a polygon. All the lines in a polygon need to be straight.
• So The Parts That Are Parallel Are The Bases That You Would Add Right?
• to find the area of a shape like this you do height times base one plus base two then you half it
• hi
For school i have to make a shape with the perimeter of 50
i have tried and tried and always got one less 49 or 1 after 51