What irrational number is between 11.1 and 11.2

Up to the table everything is right at the exception of the n value (in the table).

How many dots... figure 10: You found it it's 49 (and not 9)

How many dots... figure 25: You found it it's 109 (and not 24). Expanding it gives : 13+24x4

How many dots... figure 100: You found it it's 409 (and not 99). Expanding it gives : 13+99x4

How many common difference are added to 13 for figure 100. It's easy you also found it, it's 99, the common difference being 4 so it's 99x4

Now let's calculate the number of dots in figure "n" which is:

a(n) = a₁ + (n-1)x4 (4 being the common difference and a₁ = 1st term)

explanation: lookup again at your table and compare the FIG # part with the EXPAND part:

FIG #           EXPAND (and notice the figures in the parenthesis)
       
1                 13 + (0)x4    = 13  (for fig# 1 we have (0)
2                 13 + (1)x4    = 17  (for fig# 2 we have (1)
3                 13 + (2)x4  = 21    (for fig# 3 we have (2)
4                 13 + (3)x4  = 25    (for fig# 4 we have (3)
.                 
.                
.               
n               13 + (n-1)x4            (for fig#  we have (n-1)

As you notice the number in the parenthesis is always = fig # - 1

Hope that you understand this formula of an arithmetic progression with first term a = 13 and 4, the common difference. Note that n is the number of terms