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Tuesday, April 19, 2016

Let a,b and c be positive real numbers satisfying a+b+c=1. Prove that 9abc7(ab+bc+ca)2.

Let a,b and c be positive real numbers satisfying a+b+c=1.

Prove that 9abc7(ab+bc+ca)2.

My solution:

Schur's inequality says, for all positive real a,b and c, we have:

9abc4(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

9abc4(ab+bc+ca)1(**)

If we can prove 4(ab+bc+ca)17(ab+bc+ca)2, then we're done.

From the Cauchy-Schwarz inequality, we have:

a2+b2+c2ab+bc+ca(a+b+c)23(ab+bc+ca)

Therefore, with a+b+c=1, the above inequality becomes

13(ab+bc+ca)

Add the quantity 4(ab+bc+ca)2 to both sides we obtain:

4(ab+bc+ca)2+14(ab+bc+ca)2+3(ab+bc+ca)

4(ab+bc+ca)17(ab+bc+ca)2 (Q.E.D.)

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