a, S = 1+3+32+33+...+339
3S = 3+32+33+34+...+340
2S = 3S - S = 340 - 1
=> S = \(\frac{3^{40}-1}{2}\)
b, S = 1+3+32+33+...+339
S = (1+3+32+33)+(34+35+36+37)+....+(336+337+338+339)
S = 1(1+3+32+33) + 34(1+3+32+33) +.....+ 336.(1+3+32+33)
S = 1.40 + 34.40 +.....+ 336.40
S = 40.(1+34+...+336) chia hết cho 40
S = 4.10.(1+34+...+336) chia hết cho 4
=> Đpcm