The area of a triangle is found by taking half of the product of the base and the height. The base of the triangle can be any of the sides, but the height must be perpendicular to the base.

The perimeter of a triangle is found by adding all the sides together. So, side one plus side two plus the base, since those are the three sides.

The next lesson: Area and Circumference of a Circle

**The following transcript is provided for your convenience.**

One way to remember the formula for the area of a triangle is that a triangle is half of a parallelogram. You can imagine drawing the rest of the parallelogram out here; you can see this triangle is half of a parallelogram. The area of a parallelogram is base times height, so the area of half of a parallelogram, or a triangle, is half base times height.

Now, for example. Find the area and perimeter of the following triangle.

First, let’s find the area. We’re going to start by writing our formula for the area of a triangle. The area of a triangle is half the product of the base and the height. Now, we need to substitute our base and height. The area is 1/2, again, the base can be any of the sides, but the height must be perpendicular to it. So, start with the perpendicular height, right here, the 6 inches. It’s perpendicular to this base, 13 inches. Therefore, our base is 13 inches, and our height is 6 inches. So, we substitute 13 inches for a base, times 6 inches for our height.

Now, since all we’re doing is multiplying, we can multiply in any order. We can take half of 13 and multiply that times 6, or we can take half of 6 and multiply that times 13, or we can do 13 times 6 and then take half of that product. So, it’s whatever is easiest, and, to me, I think taking half of 6 would be the easiest, so I’m going to start by taking half of 6 inches, half of 6 inches is 3 inches, times 13 inches. 3 inches times 13 inches is 39 inches squared.

Now, for the perimeter. Again, we’re going to start with our formula. The perimeter of a triangle is the sum of all the sides. So, 10 inches plus 8 inches plus our base, 13 inches. And we can, again, add this in any order that we want, so, maybe we start with 10 inches plus 13 inches, 23 inches, plus 8 inches, 23 plus seven would 30, and eight is one more than that, so the perimeter is 31 inches.