sn=a1(1-q^n)/(1-q)a1(1-q^4)/(1-q)=1a1(1-q^8)/(1-q)=4(1-q^8)/(1-q^4)=4q^4+1=4q^4=3a1(1-3)/(1-q)=1a1/(1-q)=-1/2s12=a1(1-q^12)/(1-q)=a1/(1-q)*[1-(q^4)^3]=-1/2*(1-27)=13
