Question

Calculati suma : 1+6+11+16+...+66. 

Answer 1

Sn= (1+66)*14/2= 67*7= 469

Answer 2

S=1+6+(10+1)+(10+6)+(20+1)+(20+6)+......+(60+1)+(60+6)
Observ ca suma 1+6=7 se repeta de 7 ori iar 10 +20+30+...+60 apare de 2 ori
⇒ S=7*7+2(10+20+..+60)=49+2*10(1+2+...+6)=49+20*6*7/2=49+20*21=469
S-a folosit formula 1+2+3+....+n=n(n+1)/2