49=[(2*10+1)/3]^2;
4489=[(2*10^2+1)/3]^2;
444889=[(2*10^3+1)/3]^2;
44448889=[(2*10^4+1)/3]^2;
4444488889=[2*10^5+1)/3]^2;
…………
44…488…89=[2*10^n+1)/3]^2.n个4,n-1个8.