If 1+2=5, 2+5=12, and 3+6=21, what is 5+8=?

Answer 1

If 1+2=5, 2+5=12, and 3+6=21, what is 5+8=?
1+2x2=5, 2+2x5=12 (first two x2), 3+3x6=21, 4+3xN (second two x3) and next two seems to be x4 so 5+4x8=37

The answer is 37