Let a,b and c be positive real numbers satisfying a+b+c=1.
Prove that 9abc≥7(ab+bc+ca)−2.
My solution:
Schur's inequality says, for all positive real a,b and c, we have:
9abc≥4(ab+bc+ca)(a+b+c)−(a+b+c)3(*)
In our case, we're given a+b+c=1, so substituting that into (*) yields
9abc≥4(ab+bc+ca)−1(**)
If we can prove 4(ab+bc+ca)−1≥7(ab+bc+ca)−2, then we're done.
From the Cauchy-Schwarz inequality, we have:
a2+b2+c2≥ab+bc+ca⟹(a+b+c)2≥3(ab+bc+ca)
Therefore, with a+b+c=1, the above inequality becomes
1≥3(ab+bc+ca)
Add the quantity 4(ab+bc+ca)−2 to both sides we obtain:
4(ab+bc+ca)−2+1≥4(ab+bc+ca)−2+3(ab+bc+ca)
4(ab+bc+ca)−1≥7(ab+bc+ca)−2 (Q.E.D.)
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