 So this is section 11.2, area of triangles, trapezoids, and rhombi. In order to find the area of a triangle, you need to identify the base and the height of the triangle. Now, very often the base of the triangle will be on the bottom of the page, but it doesn't need to be. So for example, if this were the base, let's call that B, the height is always perpendicular to the base. So here would be, for example, the height of the triangle. The height is sometimes also called the altitude. In this next triangle we see here would be a base, and it appears as though the height in this triangle coincides with a full side length. So here's base and height. And then in this last triangle, this obtuse triangle, if this is our base length, the height must still be perpendicular to the base. And so if we extend the base, we see that the height of the triangle would be this perpendicular distance. So there's your height. So to find the area of a triangle, we use the formula A for area equals one-half times base times height. And I'll show you a video or two to kind of explain why we use one-half base times height. In other words, how that relates to a parallelogram. So it turns out that each of these triangles has the same area. And the reason that's true is they all have the same base length, three units, three, three units, three, three, and three. And then they all have the same height as well. This height here is, if I count boxes, one, two, three, four. One, two, three, four. Remember, within obtuse triangle, the height is perpendicular to the base. One, two, three, four. One, two, three, four. One, two, three, four. So each of these triangles has the same area. And in particular, that area is one-half base times height. One-half times three times four. So each of these triangles has an area of six square units.