AlgebraDifficulty 6.8National olympiadFind the answer
Find all pairs of real numbers k,l such that inequality ka2+lb2>c2 applies to the lengths of sides a,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. Using the Law of Cosines, we have: c2=a2+b2−2abcos∠C Substituting this into the inequality, we get: ka2+lb2>a2+b2−2abcos∠C Simplifying, we obtain: (k−1)a2+(l−1)b2>−2abcos∠C
2. **Considering the Range of cos∠C:** Since cos∠C can take values arbitrarily close to −1 (but not exactly −1 because A, B, and C would be collinear), we must have: (k−1)a2+(l−1)b2≥2ab
3. **Analyzing the Case When One of k or l is Not Larger Than 1:** Suppose k≤1. Then: (k−1)a2+(l−1)b2≤(l−1)b2 This implies: (l−1)b2≥2ab This inequality cannot hold for sufficiently large a. Therefore, both k and l must be greater than 1.
4. Ensuring the Inequality Holds: We need to ensure: (k−1)a2+(l−1)b2≥2ab(k−1)(l−1) If (k−1)(l−1)≥1, then: (k−1)a2+(l−1)b2≥2ab This condition is necessary for the initial inequality to hold.
5. Showing the Necessity of the Condition: If (k−1)(l−1)<1, then: (k−1)a2−2ab+(l−1)b2≥0 This is a quadratic inequality in a with a non-positive discriminant: D=(2b)2−4(k−1)(l−1)b2=4b2(1−(k−1)(l−1)) If (k−1)(l−1)<1, then D>0, which is a contradiction. Therefore, (k−1)(l−1)≥1 is necessary.
6. Conclusion: The suitable pairs (k,l) are those that satisfy: (k−1)(l−1)≥1andk,l>1 This can be rewritten as: kl≥k+landk,l>1
The final answer is (k,l) such that kl≥k+l and k,l>1.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: NuminaMath-1.5,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.