Question

buna, chiar am nevoie de punctul a), ma ajutati pls? ​

Answer 1

$\it s=8+8^2+8^3+8^4+\ ...\ +8^{41}+8^{42}\\ \\ s=8\cdot(1+8+8^2+8^3+\ ...\ +8^{40}+8^{41})\ \in\ M_{8}\ \Rightarrow\ 8|s\ \ \ \ \ (1)\\ \\ s=8(1+8)+8^3(1+8)+\ ...\ +8^{41}(1+8)=9\cdot(8+8^3+\ ...\ +8^{41})\Rightarrow\\ \\ \Rightarrow s\in M_9 \Rightarrow 9|s\ \ \ \ \ (2)\\ \\ (1),\ (2) \Rightarrow 72|s$