Question

2+4+6+.............+20=
(sumele lui Gauss)

Answer 1

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

Answer 2

$\displaystyle 2+4+6+...+20=2(1+2+3+...+10)=2 \times \frac{10(10+1)}{2} = \\ \\ =2 \times \frac{10 \times 11}{2} =\not2 \times \frac{110}{\not 2} =110$