Question

13, 17, 26, 42, ?,103​

Answer 1

Sequences can often be solved by inspection of the differences between each of the numbers. In this case,

13–12=1

17–13=4

26–17=9

Now we look for a pattern.

1, 4, 9 are the first three perfect squares. The next is 4^2 = 16. So X is the last number in the sequence + 16.

X = 26 + 16 = 42.

The series itself can be described as:

X(n) = 12 + Sum_From_1_to_n[ (n-1)^2 ]