`b,` `S=5+5^2+5^3+5^4+...+5^2004`
`⇒S=(5+5^3)+(5^2+5^4)+...+(5^2002+5^2004)`
`⇒S=(5+5^3)+5(5+5^3)+...+5^2002(5+5^3)`
`⇒S=130+5.130+...+5^2002.130`
`⇒S=130.(1+5+...+5^2002)`
`⇒S=65.2.(1+5+...+5^2002)⋮65` `(đpcm)`
`S=5+5^2+5^3+5^4+...+5^2004`
`⇒S=(5+5^4)+(5^2+5^5)+...+(5^2001+5^2004)`
`⇒S=5(1+5^3)+5^2(1+5^3)+...+5^2001(1+5^3)`
`⇒S=5.126+5^2.126+...+5^2001.126`
`⇒S=126.(5+5^2+...+5^2001)⋮65` `(đpcm)`
