Explicație pas cu pas:
S1=2+4+6+8+....+20=>
S1=2(1+2+3+4+...+10)=>S1=2*10*11/2=>S1=10*11
S2=1+3+5+....+19
S2=19+17+15+...+1 =>
2*S2=20+20+....+20
Ne trebuie acum cati de 20 avem dica nr de termeni a lui S2
n=(19-1)/2+1=>n=9+1=>n=10
=>2*S2=10*20=>S2=10*10
S1-S2=10*11-10*10=10(11-10)=10*1=10