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a fost răspuns

Calculati:
a)1+2+3+...50;
b)5+6+6+...55;
c)10+11+12+...60,

Răspuns :

1+2+3+...+50=50×51:2=1275
5+6+7+8+...+55=1+2+3+4+5+6+...+55-1-2-4=55×56:2-1-2-3-4=1530
10+11+12+...60=1+2+3+4+5+6+7+8+9+10+11+...+60-1-2-3-4-5-6-7-8-9=60×61:2-9×10:2=1830-45=1785

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