Fibonacci code doesn't work

[Bruno Valdeolmillos]

Hello, good day, in the tutorial I saw it says that the "algebraic" formula (I don't know if the term is correct, if it isn't, please correct me) is the one in the image.
Now, I try it in pencil and I GET AN ERROR, FOR EJ:
(8-1) + (8-2) This gives 7+6=13.  OK.
But with 13  WRONG:
(13-1) + (13-2)
  12+11=23.
DOES ANYONE KNOW WHY? Thanks

[Ron McLeod]

The next number in the sequence is calculated by adding the two numbers that proceed it, not the specific values 1 and 2.  

It appears that in the tutorial, № -1 represents the value one position to the left and № -2 represents the value two positions to the left : La sucesión comienza con los números 0 y 1, y a partir de estos cada término es la suma de los dos anteriores.

For example:
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ... 0, 1=>1   1, 1=>2      1, 2=>3         2, 3=>5            3, 5=>8               5, 8=>13                  8, 13=>21                     13, 21=>34

[Tim Cooke]

The typesetting is unfortunate. I think this is what they meant.

x_n = x_n−1 + x_n−2