11*a2(1-(q*q)^n)/(1-q*q)=a1(1-q^2n)/(1-q)a2=q*a1化简可求出q=10a3+a4=a2*a4*11→100a1+1000a1=10a1*1000a1*11→a1=1/100an=1/100*q^(n-1)=10^(n-3)
