Bài 1:
B = 3 + $3^{2}$ + $3^{3}$ + ... + $3^{120}$
B = (3 + $3^{2}$ + $3^{3}$) + ... + ($3^{118}$ + $3^{119}$ + $3^{120}$
B = 3 . (1 + 3 + $3^{2}$) + ... + $3^{118}$ . (1 + 3 + $3^{2}$)
B = 3 . 13 + ... + $3^{118}$ . 13
B = 13 . (3 + ... + $3^{118}$) $\vdots$ 13
Vậy B $\vdots$ 13
Bài 2:
C = 5 + $5^{2}$ + $5^{3}$ + ... + $5^{20}$
C = (5 + $5^{2}$ + $5^{3}$ + $5^{4}$) + ... + $5^{16}$ . (5 + $5^{2}$ + $5^{3}$ + $5^{4}$)
C = 780 + ... + 780 . $5^{16}$
C = 780 . (1 + ... + $5^{16}$)
C = 13 . 60 (1 + ... + $5^{16}$) $\vdots$ 13
Vậy B $\vdots$ 13
