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# Understand the problem

Let a, b, c be positive real numbers such that a + b + c = 1. Prove that $$\frac {a} {a^2 + b^3 + c^3} + \frac {b}{ b^2 + c^3 + a^3 } + \frac {c} { c^2 + a^3 + b^3 } \leq \frac{1}{5abc}$$

##### Source of the problem

Regional Math Olympiad, 2019 Problem 3

Inequality
6/10
##### Suggested Book
Challenges and Thrills in Pre College Mathematics

Do you really need a hint? Try it first!

The clue: Number 5 in the right-hand side!

We will be applying AM-GM inequality. But first, to get the 5 on the right, we need 5 terms in the left (or bunch of five terms).

Try to use a+b+c =1 to cook it up!

In the first term’s denominator, we have $$a^2 + b^3 + c^3$$.

Multiply 1 to $$a^2$$ that is multiply by a + b + c (nothing changes because, multiplying by does not change anything).

Hence we have $$a^2 \cdot 1 + b^3 + c^3 = a^2 ( a + b + c) + b^3 + c^3$$

Expanding we have  $$a^3 + b^3 + c^3 + a^2 b + a^2 c$$

Now apply AM – GM inequality to this we have $$\frac{ a^3 + b^3 + c^3 + a^2 b + a^2 c}{5} \geq (a^3 \cdot b^3 \cdot c^3 \cdot a^2 b \cdot a^2 c )^{1/5}$$

Therefore we have

$$a^3 + b^3 + c^3 + a^2 b + a^2 c \geq 5 \cdot (a^7 b^4 c^4)^{1/5}$$

Taking the reciprocal we have and noting that the left hand side is still $$a^2 + b^3 + c^3$$ we have

$$\frac {1} {a^2 + b^3 + c^3} \leq \frac {1}{5 \cdot (a^{7/5} b^{4/5} c^{4/5} }$$

Multiplying the numerator and denominator by a we have the desired expression in the left.

$$\frac {a} {a^2 + b^3 + c^3} \leq \frac {a}{5 \cdot (a^{7/5} b^{4/5} c^{4/5} }$$

Simplifying

$$\frac {a} {a^2 + b^3 + c^3} \leq \frac {1}{5 \cdot (a^{2/5} b^{4/5} c^{4/5} }$$

Now try computing the same for the other two terms on the left.

$$\frac {a} {a^2 + b^3 + c^3} \leq \frac {1}{5 \cdot (a^{2/5} b^{4/5} c^{4/5} }$$

$$\frac {b} {b^2 + c^3 + a^3} \leq \frac {1}{5 \cdot (a^{4/5} b^{2/5} c^{4/5} }$$

$$\frac {c} {c^2 + a^3 + b^3} \leq \frac {1}{5 \cdot (a^{4/5} b^{4/5} c^{2/5} }$$

$$\frac {a} {a^2 + b^3 + c^3} + \frac {b} {b^2 + c^3 + a^3} + \frac {c} {c^2 + a^3 + b^3} \leq \frac {1}{5 \cdot (a^{2/5} b^{4/5} c^{4/5} } + \frac {1}{5 \cdot (a^{4/5} b^{2/5} c^{4/5} } \frac {1}{5 \cdot (a^{4/5} b^{4/5} c^{2/5} }$$

Now apply AM- GM Inequality one more time to the left hand term.

$$\frac{ \frac {1}{5 \cdot (a^{2/5} b^{4/5} c^{4/5} } + \frac {1}{5 \cdot (a^{4/5} b^{2/5} c^{4/5} } + \frac {1}{5 \cdot (a^{4/5} b^{4/5} c^{2/5} }}{3} \leq ( \frac {1}{5 \cdot (a^{2/5} b^{4/5} c^{4/5} } \cdot \frac {1}{5 \cdot (a^{4/5} b^{2/5} c^{4/5} } \cdot \frac {1}{5 \cdot (a^{4/5} b^{4/5} c^{2/5} })^{1/3}$$

Simplifying  we have

$$\frac{ \frac {1}{5 \cdot (a^{2/5} b^{4/5} c^{4/5} } + \frac {1}{5 \cdot (a^{4/5} b^{2/5} c^{4/5} } + \frac {1}{5 \cdot (a^{4/5} b^{4/5} c^{2/5} }}{3} \leq ( \frac {1}{5^3 \cdot (a^{10/5} b^{10/5} c^{10/5} } )^{1/3}$$

Simplifying further we have

$$\frac {1}{5 \cdot (a^{2/5} b^{4/5} c^{4/5} } + \frac {1}{5 \cdot (a^{4/5} b^{2/5} c^{4/5} } + \frac {1}{5 \cdot (a^{4/5} b^{4/5} c^{2/5} } \leq 3 \cdot \frac {(abc)^{1/3}}{5abc}$$

Finally we know that $$3 \cdot (abc)^{1/3} \leq 1$$. Why? Apply AM-GM to a, b, c

$$\frac{a+b+c}{3} \geq (abc)^{1/3}$$

Since a + b + c = 1 we have the result.

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