Let a,\,b and c be positive real numbers satisfying a+b+c=1.
Prove that 9abc\ge7(ab+bc+ca)-2.
My solution:
Schur's inequality says, for all positive real a,\,b and c, we have:
9abc\ge 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\ge 4(ab+bc+ca)-1(**)
If we can prove 4(ab+bc+ca)-1\ge 7(ab+bc+ca)-2, then we're done.
From the Cauchy-Schwarz inequality, we have:
a^2+b^2+c^2\ge ab+bc+ca\implies (a+b+c)^2\ge 3(ab+bc+ca)
Therefore, with a+b+c=1, the above inequality becomes
1\ge 3(ab+bc+ca)
Add the quantity 4(ab+bc+ca)-2 to both sides we obtain:
4(ab+bc+ca)-2+1\ge 4(ab+bc+ca)-2+3(ab+bc+ca)
4(ab+bc+ca)-1\ge 7(ab+bc+ca)-2 (Q.E.D.)
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