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Tuesday, April 19, 2016

Let a,\,b and c be positive real numbers satisfying a+b+c=1. Prove that 9abc\ge7(ab+bc+ca)-2.

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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