已知 x<0; f(x)>1 >0
x=0时 f(0)≠0 ;f(0+0)=f(0)f(0)=>f(0)=1>0
当 x>0 时 : -x<0 =>f(-x)>1 但 f(0)=f(x+(-x))=f(x)f(-x)=1 =>f(x);f(-x)同号 =>0
x>0(-x<0) ;a-x0
=> f(x)减函数.....(2)
f(4)=1/16= f(2+2)=f(2)f(2)=>f(2)=1/4=f(1+1)=f(1)f(1)=>f(1)=1/2
f(5)=f(1+4)=f(1)f(4)=1/32 =>
f(x-3)f(x-5)=f(2x-8)=f(2x)f(-8)≤ 1/4
f(-8)= 1/f(8) ;f(8)=f(4+4)=f(4)f(4)=1/256 => f(-8)=256
=>f(2x)≤1/1024 =>f(x)f(x)≤1/1024 => f(x) ≤1/32=f(5) => x≥5...(3)
(1)(2)很简单，(3)f(2x-8)<=f(2),2x-8>=2,x>=5
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