a) 2 +$2^{2}$+$2^{3}$ +...+$2^{119}$+$2^{120}$
=2.(1+2)+$2^{3}$.(1+2)+.........+$2^{119}$.(1+2)
=2.3+$2^{3}$.3+...+$2^{119}$.3
=3.(2+$2^{3}$+...+$2^{119}$) ⋮ 3
b)
2 +$2^{2}$+$2^{3}$ +...+$2^{119}$+$2^{120}$
=2.(1+2+4)+$2^{4}$.(1+2+4)+.........+$2^{118}$.(1+2+4)
=2.7+$2^{4}$.7+...+$2^{118}$.3
=7.(2+$2^{4}$+...+$2^{118}$) ⋮ 3