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a) 10+15+20....+2010-9-13-17-.....-1609

Răspuns :

Adrianhorea1

(10+15+...+2010)-(9+13+...+1609)=

=[5(2+3+...+402)]-[(4×2+1)+(4×3+1)+...+(4×402+1)}=

=(5×81002)-[4×(2+3+..+402)+401}

=5×81002- 4×81002-401=

=81002-401=

=80601

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