Question

determine the value of x and y for the sequence : 3;x;11;21;y

Answer 1

testing for AP:

x-3 = 11-x
2x = 14
x = 7, but then this is contradicted in 11 ---> 21

testing for GP:
x/3 = 11/x
x^2 = 33
x = √33 , but this is contradicted in 11 ----> 21

so the sequence is neither arithmetic, nor geometric

I can't see any obvious pattern

Answer 2

3+4 = 7

7+4 = 11
11+10 = 21
21+10 = 31