Maths Olympiad Prep

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Algebra Difficulty 6.8 National olympiad Find the answer

Find all pairs of real numbers k,lk, l such that inequality ka2+lb2>c2ka^2 + lb^2> c^2 applies to the lengths of sides a,b,ca, b, c of any triangle.

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

Solution

1. Applying the Law of Cosines:
The given inequality is ka2+lb2>c2 ka^2 + lb^2 > c^2 . Using the Law of Cosines, we have:
c2=a2+b22abcosC c^2 = a^2 + b^2 - 2ab \cos{\angle{C}}
Substituting this into the inequality, we get:
ka2+lb2>a2+b22abcosC ka^2 + lb^2 > a^2 + b^2 - 2ab \cos{\angle{C}}
Simplifying, we obtain:
(k1)a2+(l1)b2>2abcosC (k-1)a^2 + (l-1)b^2 > -2ab \cos{\angle{C}}

2. **Considering the Range of cosC\cos{\angle{C}}:**
Since cosC\cos{\angle{C}} can take values arbitrarily close to 1-1 (but not exactly 1-1 because AA, BB, and CC would be collinear), we must have:
(k1)a2+(l1)b22ab (k-1)a^2 + (l-1)b^2 \geq 2ab

3. **Analyzing the Case When One of kk or ll is Not Larger Than 1:**
Suppose k1k \leq 1. Then:
(k1)a2+(l1)b2(l1)b2 (k-1)a^2 + (l-1)b^2 \leq (l-1)b^2
This implies:
(l1)b22ab (l-1)b^2 \geq 2ab
This inequality cannot hold for sufficiently large aa. Therefore, both kk and ll must be greater than 1.

4. Ensuring the Inequality Holds:
We need to ensure:
(k1)a2+(l1)b22ab(k1)(l1) (k-1)a^2 + (l-1)b^2 \geq 2ab \sqrt{(k-1)(l-1)}
If (k1)(l1)1(k-1)(l-1) \geq 1, then:
(k1)a2+(l1)b22ab (k-1)a^2 + (l-1)b^2 \geq 2ab
This condition is necessary for the initial inequality to hold.

5. Showing the Necessity of the Condition:
If (k1)(l1)<1(k-1)(l-1) < 1, then:
(k1)a22ab+(l1)b20 (k-1)a^2 - 2ab + (l-1)b^2 \geq 0
This is a quadratic inequality in aa with a non-positive discriminant:
D=(2b)24(k1)(l1)b2=4b2(1(k1)(l1)) D = (2b)^2 - 4(k-1)(l-1)b^2 = 4b^2(1 - (k-1)(l-1))
If (k1)(l1)<1(k-1)(l-1) < 1, then D>0D > 0, which is a contradiction. Therefore, (k1)(l1)1(k-1)(l-1) \geq 1 is necessary.

6. Conclusion:
The suitable pairs (k,l)(k, l) are those that satisfy:
(k1)(l1)1andk,l>1 (k-1)(l-1) \geq 1 \quad \text{and} \quad k, l > 1
This can be rewritten as:
klk+landk,l>1 kl \geq k + l \quad \text{and} \quad k, l > 1

The final answer is (k,l) \boxed{ (k, l) } such that klk+lkl \geq k + l and k,l>1k, l > 1.

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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.