Analysis 1
b−1+c1=b(1−b1+bc1)=b(1+a−b1),
Therefore,
(a−1+b1)(b−1+c1)=b(a−1+b1)(a+1−b1)=b[a2−(1−b1)2]⩽ba2,
Similarly, (b−1+c1)(c−1+a1)⩽cb2,(c−1+a1)(a−1+b1)⩽ac2. If a−1+b1,b−1+c1,c−1+a1 are not all positive, without loss of generality, assume a−1+b1⩽0, then a⩽1−b10,c−1+a1>0, hence the proposition holds.
If a−1+b1,b−1+c1,c−1+a1 are all positive, then from the above three inequalities we get
(a−1+b1)2(b−1+c1)2(c−1+a1)2⩽a3b3c3=1,(a−1+b1)(b−1+c1)(c−1+a1)⩽1.
Analysis 2
By abc=1, without loss of generality, let a=yx,b=zy,c=xz, where x,y,z∈R+. Then the inequality to be proved is
(yx−1+yz)(zy−1+zx)(xz−1+xy)⩽1,
which is
(x+y−z)(y+z−x)(z+x−y)⩽xyz.
Since the sum of any two of x+y−z,y+z−x,z+x−y is positive, at most one of them can be negative. If exactly one is negative, then (1) holds; if all three are non-negative, then
x2⩾x2−(y−z)2=(x+y−z)(x+z−y),y2⩾(y+z−x)(x+y−z),z2⩾(z+x−y)(y+z−x),
Multiplying these three inequalities and taking the square root, we get (x+y−z)(y+z−x)(z+x−y)⩽xyz. Hence the proposition is proved.