(9^n*27^n-1)/(3^3n+1)=81即[3^(2n+3n-3)]/[3^(3n+1)]=81即3^(2n-4)=3^4∴2n-4=4 n=4（n^3)^2/[(n^4)^3/(n^3)^3]=（n^6)/(n^12/n^9)=（n^6)/(n^3)=n^3=4^3=64
