an = 2n+1, n∈N , n≥0
2n =2*1 =2 e chiar ratia
deci a15 = a1+(15-1) *2=1+2*14=29
S15 = 1+3+5+....+25+27+29= (1+29)+(3+27)+...+(13+17)+15 =
30 *7+30/2=30*7,5=225
altfel
1+2*1+1+2*2+1+...2+14+1= 1+1+...+1(sunt 15de 1)+ +2(1+2+...+14)=1*15+2*14*15/2 = 15+14*15=15*15=225
