Question

calculati suma 5+6+7+....+81

Answer 1

$\rm S=5+6+7+...+81\\\\Putem\ scrie\ suma\ S\ si\ astfel:\ \\\\S=(1+2+3+...+81)-(1+2+3+4)\\\\Aplicam\ Suma\ Lui\ Gauss:\ \\\\S=81\cdot82\div2-10\\\\S=81\cdot41-10\\\\\boxed{S=3311}$

Answer 2

S=5+6+7+...+81=(1+2+3+...+81)-(1+2+3+4) pentru ca de la 1 la 5 lipsesc numerele care ar completa suma gauss
S=82x81/2-10=41x81-10=3321-10=3311