Question

1+5+5^2+...+5^101:3

Answer 1

1+5+5²+...+5^101 =

= (1+5)+(5²+5³)+...+(5^100+5^101) =

= (1+5)+5²(1+5)+5⁴(1+5)+...+5^98(1+5) = 5^100(1+5) =

= 6+5²•6+5⁴•6+...+5^100•6 =

= 6(1+5²+5⁴+...+5^100) =

=> 6(1+5²+5⁴+...+5^100) : 3 =

= 2(1+5²+5⁴+...+5^100)