Ta có : x=[TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 31 \end{matrix}[/TEX]
y = [TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 77 \end{matrix}[/TEX]
= [TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 31 \end{matrix}[/TEX].1[TEX] \begin{matrix} \underbrace{ 0000\cdots000} \\ 46 \end{matrix}[/TEX]+[TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 46 \end{matrix}[/TEX]
Vì [TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 31 \end{matrix}[/TEX] chia 3 dư 1
=> [TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 31 \end{matrix}[/TEX] = 3k+1(k thuộc N)
Vì [TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 46 \end{matrix}[/TEX] chia 3 dư 1
=> [TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 46 \end{matrix}[/TEX] = 3m+1(m thuộc N)
Vì 1[TEX] \begin{matrix} \underbrace{ 0000\cdots000} \\ 46 \end{matrix}[/TEX] chia 3 dư 1
=> 1[TEX] \begin{matrix} \underbrace{ 0000\cdots000} \\ 46 \end{matrix}[/TEX] = 3n+1 (n thuộc N)
Xét hiệu : xy-2
= [TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 31 \end{matrix}[/TEX].[TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 31 \end{matrix}[/TEX].1[TEX] \begin{matrix} \underbrace{ 0000\cdots000} \\ 46 \end{matrix}[/TEX]+[TEX] \begin{matrix} \underbrace{ 1111\cdots11111} \\ 31 \end{matrix}[/TEX]. [TEX] \begin{matrix} \underbrace{ 1111\cdots1111} \\ 46 \end{matrix}[/TEX] -2
= (3k+1)(3k+1)(3n+1)+(3k+1)(3m+1)-2
= (3k+1)(9kn+3k+3n+1+3m+1)-2
= (3k+1)(9kn+3k+3m+3n+2)-2
= (3k+1)(9kn+3k+3m+3n)+2(3k+1)-2
= 3.(3k+1)(3kn+k+m+n)+2.(3k+1)-2
= 3.(3k+1)(3kn+k+m+n)+6k +2-2
= 3.(3k+1)(3kn+k+m+n)+6k
=> Chia hết cho 3
Vậy x.y-2 chia hết cho 3(đpcm)