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

Răspuns :

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[tex](3+6+9+....+30)=3(1+2+3+....+10)=3 \frac{10.11}{2}= \frac{330}{2}=165[/tex]
[tex](2+4+6+....+20)=2(1+2+3+.....+10)=2 \frac{11.10}{2}= \frac{220}{2}=110[/tex]
165-110=55

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