 Here, we want to find the area of an equilateral triangle. If one side length is 5, PR, then all side lengths are 5. And so if I want to find the height of the triangle, in other words, I want to find this length, what I can do is I can split up the triangle into two. That's kind of poorly drawn. What I can do is I can split this triangle up into two right triangles. In a particular, if we remember back to a previous unit, equilateral triangles have 60-degree angles. And so when I split an equilateral triangle up, I create a 30-60-90 triangle on one side and a 30-60-90 on the other side. Now, 5 refers to the 2n side, and so half of a side length would be half of 5, and the height, or the altitude, would be 1 half times 5 times root 3. And so the area of this triangle, area is 1 half base times height, base times height, and if we use 5 as the base, times 5 times the height, which was this length, times half times 5 times root 3, we can simplify the calculations a little bit. If I bring together the 2n halves, I have 1 half times 1 half times 5 times 5 times root 3. And that simplifies one step further to 1 fourth times 25 times root 3, or 25 fourths squared 3. And so we see 25 is 5 squared. And so the area formula for any equilateral triangle is 1 fourth times the side length squared times the square root of 3. And that works for any equilateral triangle, just 1 fourth times the side length squared times root 3.