Răspuns :
a) 1+2+3+....+80= 80*81/2=3240
b) 2+4+6...+100 =2*(1+2+3....+50)=2*50*51/2=2550
unde : 1+2+3....+n= n+(n+1)/2
b) 2+4+6...+100 =2*(1+2+3....+50)=2*50*51/2=2550
unde : 1+2+3....+n= n+(n+1)/2