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2+4+6+8+10+.........+20=?

Răspuns :

[tex]2+4+6+8+...+20= \\ \\ =2(1+2+3+4+...+10)= \\ \\ =2 \cdot \frac{10 \cdot 11}{2}= \\ \\ =10 \cdot 11= \\ \\ =110. [/tex]