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Algebra Difficulty 7.8 National olympiad, round 2 Find the answer

Let a,b,c,da, b, c, d three strictly positive real numbers such that a2+b2+c2=d2+e2,a^{2}+b^{2}+c^{2}=d^{2}+e^{2}, a4+b4+c4=d4+e4.a^{4}+b^{4}+c^{4}=d^{4}+e^{4}. Compare a3+b3+c3a^{3}+b^{3}+c^{3} with d3+e3,d^{3}+e^{3},

A number or a short expression. Spacing and $ signs are ignored.

Solution

Given three strictly positive real numbers a,b,c a, b, c and two strictly positive real numbers d,e d, e such that:

a2+b2+c2=d2+e2, a^{2} + b^{2} + c^{2} = d^{2} + e^{2},
a4+b4+c4=d4+e4. a^{4} + b^{4} + c^{4} = d^{4} + e^{4}.

We need to compare a3+b3+c3 a^{3} + b^{3} + c^{3} with d3+e3 d^{3} + e^{3} .

### Step 1: Use the Known Equalities

We have been given the equalities involving squares and fourth powers:

1. a2+b2+c2=d2+e2 a^{2} + b^{2} + c^{2} = d^{2} + e^{2} — This tells us that the sum of the squares of a,b,c a, b, c equal those of d,e d, e .
2. a4+b4+c4=d4+e4 a^{4} + b^{4} + c^{4} = d^{4} + e^{4} — This similarly extends to the fourth powers.

### Step 2: Apply Power Mean Inequality

The Power Mean Inequality states that for any non-negative numbers x1,x2,,xn x_1, x_2, \ldots, x_n and any real numbers p<q p < q ,

(x1p+x2p++xnpn)1/p(x1q+x2q++xnqn)1/q. \left( \frac{x_1^p + x_2^p + \cdots + x_n^p}{n} \right)^{1/p} \le \left( \frac{x_1^q + x_2^q + \cdots + x_n^q}{n} \right)^{1/q}.

#### Applying the Inequality:

- Let us apply this inequality to the sequences a,b,c a, b, c and d,e d, e with p=3 p = 3 and q=4 q = 4 .
- For a,b,c a, b, c :

(a3+b3+c33)1/3(a4+b4+c43)1/4. \left( \frac{a^3 + b^3 + c^3}{3} \right)^{1/3} \le \left( \frac{a^4 + b^4 + c^4}{3} \right)^{1/4}.

- For d,e d, e :

(d3+e32)1/3(d4+e42)1/4. \left( \frac{d^3 + e^3}{2} \right)^{1/3} \le \left( \frac{d^4 + e^4}{2} \right)^{1/4}.

### Step 3: Analyzing Our Equations

Given a4+b4+c4=d4+e4 a^4 + b^4 + c^4 = d^4 + e^4 , the right sides of the inequalities are equal, which gives us:

(a4+b4+c43)1/4=(d4+e42)1/4. \left( \frac{a^4 + b^4 + c^4}{3} \right)^{1/4} = \left( \frac{d^4 + e^4}{2} \right)^{1/4}.

Since a3+b3+c33 \frac{a^3 + b^3 + c^3}{3} measure follows the cube root mean of smaller numbers due to the positive nature and similar inequality holds for such values compared to fewer larger cubes in d3+e3 d^3 + e^3 :

d3+e3a3+b3+c3. d^3 + e^3 \ge a^3 + b^3 + c^3.

Thus, the comparison yields:

d3+e3a3+b3+c3. \boxed{d^3 + e^3 \ge a^3 + b^3 + c^3}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.