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calculati (3+6+9+....+30)-(2+4+6....20)

Răspuns :

(3+6+9+...+30)-(2+4+6...20)

=33×5-22×5
=165-110
=55
3+6+9+...+30= 3(1+2+3+...+10)=3·[(10·11):2]=3·(110:2)=3·55=165
2+4+6+...+20=2(1+2+3+...+10)=2·[(10·11):2]=2·(110:2)=2·55=110
165-110=55