Recall now the well-known inequality (x+y+z)2≥3(xy+yz+zx) and set x=ab, y=bc, z=ca, to obtain (ab+bc+ca)2≥3abc(a+b+c)=9abc where we have used a+b+c=3. By taking the square roots on both sides of the last one we obtain: ab+bc+ca≥3abc.(1) Also by using AM-GM inequality we get that abc1+1≥2abc1.(2) Multiplication of (1) and (2) gives (ab+bc+ca)(abc1+1)≥3abc⋅2abc1=6. So A≥2⋅6−9=3 and the equality holds if and only if a=b=c=1, so the minimum value is 3.
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