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Algebra Difficulty 6.9 National olympiad Find the answer

Let a1,a2,,a1000a_1,a_2,\cdots,a_{100}\geq 0 such that max{ai1+ai,ai+ai+1}i\max\{a_{i-1}+a_i,a_i+a_{i+1}\}\geq i for any 2i99.2\leq i\leq 99. Find the minimum of a1+a2++a100.a_1+a_2+\cdots+a_{100}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Given the sequence a1,a2,,a1000a_1, a_2, \ldots, a_{100} \geq 0 such that max{ai1+ai,ai+ai+1}i\max\{a_{i-1} + a_i, a_i + a_{i+1}\} \geq i for any 2i992 \leq i \leq 99, we need to find the minimum value of a1+a2++a100a_1 + a_2 + \cdots + a_{100}.

1. Analyzing the given condition:
max{ai1+ai,ai+ai+1}i \max\{a_{i-1} + a_i, a_i + a_{i+1}\} \geq i
This implies that either ai1+aiia_{i-1} + a_i \geq i or ai+ai+1ia_i + a_{i+1} \geq i (or both).

2. Constructing a sequence that satisfies the condition:
Let's consider a sequence where ai=i2a_i = \frac{i}{2} for i=1,2,,100i = 1, 2, \ldots, 100.

3. Verifying the sequence:
For 2i992 \leq i \leq 99:
ai1+ai=i12+i2=2i12 a_{i-1} + a_i = \frac{i-1}{2} + \frac{i}{2} = \frac{2i-1}{2}
ai+ai+1=i2+i+12=2i+12 a_i + a_{i+1} = \frac{i}{2} + \frac{i+1}{2} = \frac{2i+1}{2}
We need to check if:
max{2i12,2i+12}i \max\left\{\frac{2i-1}{2}, \frac{2i+1}{2}\right\} \geq i
Clearly, 2i+12i\frac{2i+1}{2} \geq i for all ii, since:
2i+12=i+12i \frac{2i+1}{2} = i + \frac{1}{2} \geq i

4. Calculating the sum:
a1+a2++a100=12+22++1002=1+2++1002 a_1 + a_2 + \cdots + a_{100} = \frac{1}{2} + \frac{2}{2} + \cdots + \frac{100}{2} = \frac{1 + 2 + \cdots + 100}{2}
Using the formula for the sum of the first nn natural numbers:
1+2++100=1001012=5050 1 + 2 + \cdots + 100 = \frac{100 \cdot 101}{2} = 5050
Therefore:
a1+a2++a100=50502=2525 a_1 + a_2 + \cdots + a_{100} = \frac{5050}{2} = 2525

5. Checking for a lower sum:
We need to check if there is a sequence that gives a lower sum while satisfying the condition. Consider the sequence where ai=0a_i = 0 for all ii. This sequence does not satisfy the condition since max{ai1+ai,ai+ai+1}=0\max\{a_{i-1} + a_i, a_i + a_{i+1}\} = 0 which is not i\geq i.

Thus, the minimum sum of a1+a2++a100a_1 + a_2 + \cdots + a_{100} that satisfies the given condition is 25252525.

The final answer is 2525\boxed{2525}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.