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a fost răspuns

1) 1-2+3-4+5-6+.........+19-20
2) 3*(-1)+4+5+6

Răspuns :

TheCoder
1)1+3+5+...19; r=2, a1=1,an=19,n=10=> s= (1+19)*10/2=100
-2-4-6-...-20=96
1-2+3-4+5-6+.........+19-20=100-96=4
-3+4+5+6=12