If you are reffering to this dot plot, it would be 13 trees being studied.
The first is 4
The second one is
4=x-4
-x=-4-4
X=8
The first two terms are given,

and

.
The third term, according to the formula, is

and the next two terms are

Irrational.
Rational means that you can write it as a fraction, you cannot express pi as a fraction therefore it’s not rational