这个题目非常好
证明补细
1、2<=n<=4
A=a1*10^(n-1)+a2*10^(n-2)+a3*10^(n-3)……+an
a1*10^(n-1)<=A=1.5P(A)=1.5a1*a2*.....*an<=1.5*a1*9^(n-1)
(10/9)^(n-1)<=1.5
n<=log1.5/( log10-log9)+1
n<=4.8
n=1, 显然1.5P(A)不等于A
2、n=4,
1.5*a1*a2*a3*a4=1000a1+100a2+10a3+a4>=1000a1
a2*a3*a4>=667, a2,a3,a4>=667/81>8.2 ->a2=a3=a4=9
代入得 93.5a1=999, a1>10
3、n=3
1.5*a1*a2*a3=100a1+10a2+a3>=100a1
a2*a3>=67, a2,a3>=67/9>7.4 ->a2=a3=9或a2=8,a3=9或a2=9,a=8
分别代入得 36.5a1=99, a1非整数, 8a1=98或89, a1均非整数
4、n=2
1.5*a1*a2=10a1+a2>=10a1
a2>=7
a2=7, 0.5a1=7 a1=14>9 ; a2=8, 2a1=8, a1=4; a2=9, 3.5a1=9 a1=18/7非整数
故A有唯一解48