Question

1+2+3+........+1199+1200=

Answer 1

 = (  1         +    1200  ) · 1200  / 2 = 1201  · 600 = 720600

Answer 2

$\displaystyle 1+2+3+...+1199+1200= \frac{1200(1200+1)}{2} = \\ \\ = \frac{1200 \cdot 1201}{2} = \frac{1441200}{2} =720600$