a) `M=5+5^2+5^3+........+5^{80}`
`M=(5+5^2)+(5^3+5^4)+..............+(5^{79}+5^{80})`
`M=5.(1+5)+5^3. (1+5)+..............+5^{79}. (1+5)`
`M=5.6+5^3. 6+...........+5^{79}. 6`
`M=6. (5+5^3+..........+5^{79})\vdots 6`
b) `M=5+5^2+5^3+........+5^{80}`
`M=(5^{81}-5)/4`
Do $5^{81}-5\not\vdots 4$
$⇒M\not\in \Bbb N$
$⇒M$ ko là SCP