Salut!
1/(1x4) = 2/(2x4) = 1/2 - 1/4;
1/(2x6) = 2/(4x6) = 1/4 - 1/6;
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
1/(49x100)_ = 2/(98x100) = 1/98 - 1/100;
Adunam aceste relatii matematice, una sub alta, si obtinem A = 1/2 - 1/100 = 49/100;
Bafta!