Question

chứng tỏ:A=1+3+3^2+...+3^10+3^11 chia hết cho cả 5 và 8,B=1+5+5^2+...+5^7+5^8 chia hết cho 31

 

Answer 1

Ta có:

\(A=1+3+3^2+...+3^{10}+3^{11}\)

\(A=\left(1+3+3^2+3^3\right)+...+\left(3^8+3^9+3^{10}+3^{11}\right)\)

\(A=40+...+3^8.\left(1+3+3^2+3^3\right)\)

\(A=40+...+3^8.40\)

\(A=40.\left(1+...+3^8\right)\)

Vì \(40⋮5\) và \(8\) nên \(40.\left(1+...+3^8\right)⋮5\) và \(8\)

Vậy \(A⋮5\) và \(8\)

_________

Ta có:

\(B=1+5+5^2+...+5^7+5^8\)

\(B=\left(1+5+5^2\right)+...\left(5^6+5^7+5^8\right)\)

\(B=31+...+5^6.\left(1+5+5^2\right)\)

\(B=31+...+5^6.31\)

\(B=31.\left(1+...+5^6\right)\)

Vì \(31⋮31\) nên \(31.\left(1+...+5^6\right)⋮31\)

Vậy \(B⋮31\)

\(#WendyDang\)