（3n-1）（3n+2）（3n+5）（3n+8)+81=[（3n-1）（3n+8）][（3n+2)(3n+5)]+81=(9n²+21n-8)(9n²+21n+10)+81=(9n²+21n)²-2(9n²+21n)-80+81=(9n²+21n)²-2(9n²+21n)+1=(9n²...