prog. geom .
bā + bā + bā = 2 + 6 + 18 = 26
bā + bā = 8
bā - bā = 4
--------------------------------
bā + bā + bā - bā = 8 + 4 ; 2bā=12 ; bā = 6
bā = 8 - bā = 8 - 6 = 2
q = bā / bā = 6 / 2 = 3
prog. geom : 2 ; 6 ; 18 ; 54 ........