Question

cum se calculeaza 2+4+6+..+18+20

Answer 1

din doi in doi 2+4+6+8+10+12+14+16+18+20=110

Answer 2

$\displaystyle 2+4+6+...+18+20=2(1+2+3+...+9+10)= \\ \\ =2 \times \frac{10(10+1)}{2} =2 \times \frac{10 \times 11}{2} =\not 2 \times \frac{110}{\not 2} =110$