Given set=11,8,21,16,26,32,4
Given an arithmetic series and a geometric series having the first four terms from the given set.
Then,
Geometric series:4,8,16,32,64,128,256,512,1024,2048,4096,8192
Arithmetic series:11,16,21,26,31,36,...,256,....4096,...
Tn(G.P.)=4⋅2n−1
Tn(A.P.)=11+5(n−1)=5n+6
For common terms, 4⋅2n−1=5n+6
So, common terms are 16,256,4096 only.