Răspuns :
ab+ba+ac+ca+bc+cb=
10a+b+10b+a+10a+c+10c+a+10b+c+10c+b=
22a+22b+22c se divide cu 11
10a+b+10b+a+10a+c+10c+a+10b+c+10c+b=
22a+22b+22c se divide cu 11
A×10+b+b×10+a+10×a+c+c×10+a+b×10+c+c×10+b=22a+22b+22c=22×(a+b+c) divizibil cu 11