Question

Calculează suma: 3+6+9.........+78+81+84 plssssss COROANAAAAA

Answer 1

3+6+9.........+78+81+84=3(1+2+3+......26+27+28)

(1+2+3+......26+27+28) - este suma Gauss, incepe din 1 si are termenii consecutivi
are formula de calcul [n(n+1)]:2  unde n- ultimul termen

(1+2+3+......26+27+28) =(28×29):2=406

3+6+9.........+78+81+84=3(1+2+3+......26+27+28)=3×406=1218

deci  3+6+9.........+78+81+84=1218

Answer 2

3x(1+2....+28)=
3x28x29:2=1218