Question

-2-4-6-....-200
Calculati:

Answer 1

-2-4-6-....-200= -2(1+2+...+100)= -2*100*101/2= -10100

Answer 2

$\displaystyle -2-4-6-....-200= \\ \\ \text{Factor comun pe 2} \\ -2(1+2+3+...+100)=\\ \\ \text{Asupra parantezei se aplica formula:} \\ \\ \frac{n(n+1)}{2} \ \text{unde n este ultimul nr. din sir} \\ \\ \\\\ \frac{100(100+1)}{2}= \frac{\not100\cdot101}{\not2} = 50\cdot101=5050 \\ \\ \bold{-2\cdot5050=-10100}$