Question

[phan thi phuong anh]

a) Chung minh rang: ab + ba chia het cho 11

                               ab - ba chia het cho 9 (a lon hon hoac bang b)

Answer 1

[Nguyễn Hữu Thế]

Ta có: ab+ba=a.10+b+b.10+a

=> ab+ba=a.11+b.11

=> ab+ba=11.(a+b)

=> ĐPCM

Ta có: ab-ba=a.10+b-b.10+a

=> ab-ba=a.10-a+b.10-b

=> ab-ba=a.9-b.9

=> ab-ba= 9.(a-b)

=> ĐPCM