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demonstrati ca daca a+c=b+d,atunci nr. abcd cu bara se divide cu 11.
COROANA!


Răspuns :

abcd=1000a+100b+10c+d=
(1001a -a)+(99b+b)+(11c-c)+d=
(1001a+99b+11c)+(-a+b-c+d)=
11(91a+9b+c)+(b+d)-(a+c)=

( b+d=a+c, deci (b+d)-(a+c)=0 )

=11(91a+9b+c)=m11 (m=multiplu), deci abcd se divide cu 11