Calculati :-4-8-12-...-80= ? Va roggg

Răspuns :

-4-8-12-...-80= ?     
 1+2+3+....+n=n(n+1)/2   Suma Gauss

-4(1+2+3+.....+20)= -4 
× [20(20+1)/2] = - 4 ×(10×21) = - 4 × 210 = - 840