Question

Fie numerele a=3+ 6+ 9 +...+60,b=2+ 5+ 8+ ..+ 59,c=1+ 4 +7+ ... +58.determinati numarul d=(a+ b+ c):(61-55)-(a-b):20x(a-c):4.

Answer 1

Intai aflam numerelr:
a=3+6+9+...+60
Observam ca putem da factor comun pe 3:
a=3*(1+2+3+...20)
In paranteza folosin formula:
1+2+3+...+n=[n*(n+1)]/2
a=3*[20*(20+1)]/2
a=3*(10*21)
a=3*210
a=630