Question

Calculati suma numerelor naturale de 3 cifre care impartite la 12 dau restul 7

Answer 1

100:12=8,33
9*12+7=115

999 : 12=83,25
82*12+7=991

nr sunt: {115,127,139,....991}
avem 82-9+1=74 numere
suma lor 
115+127+....991 =518+40404=40922
7*74=518
12(9+10+....+82)=12[(9*74)+(1+2+...73)]=12*666+12*73(73+1)/2]=
=12*666+12*73*37=7992+32412=40404