As you may notice, as you do the samething to every basic shape, the value increases 1, from the values 2.5, 3.5, and 4.5. Now, add all those values up, and you will get 10.5. Now, divide that by 3, since that is how many numbers there are, and you will get 3.5, which is the value of the triangle.
Now, let me show you why, with the diagram below:
What you also can do is add up all the values, or 10, 21, and 36, you will get 67, which then you divide by 3 to get 22.33333333333333333333333, and then subtract this number by 21, and then use this number to go through the process of getting the golden ratio, and you also get 1.618, or the approximation.