Răspuns:
D₁₈={1 ,2 ,3 ,6 ,9, 18}
x+1=1=>x=1-1=>x=0
x+1=2=>x=2-1=>x=1
x+1=3=>x=3-1=>x=2
x+1=6=>x=6-1=>x=5 nu convine
x+1=9=>x=7 nu convine
x+1=18=>x=17 nu convine
x∈{0,1,2}
___________
M₂₁=21,42,63,84,105,126....
x+2=21=>x=21-2=>x=19
x+2=42=>x=40
x=19,40,61,82,103,124...
