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Operatii cu numere nat. 2+4+6+...+1980

Răspuns :

2+4+6+...+1980=2×(1+2+3+...+990)=2×990×991:2=990×991=981090

2+4+6+...1980 = 2(1+2+3+...+990) = 2*990*991/2=990*991= 981090