Inequality Problem - Didn't use 2015 to avoid Complicacy!

Algebra Level 5

a b c ( a 125 + b 125 + c 125 ) 16 K ( a 2003 + b 2003 + c 2003 ) \large{abc \left( a^{125} + b^{125} + c^{125} \right)^{16} \leq K \left( a^{2003} + b^{2003} + c^{2003} \right)}

Find the value of the smallest constant K K such that, for any positive real numbers a , b , c a,b,c , the above inequality satisfies. Submit the value of ln ( K ) \ln(K) correct upto two decimal places as your answer.


Bonus: Can you find the smallest constant K K in terms of ( p , q , n ) Z (p,q,n) \in \mathbb Z for the inequality below?

( i = 1 n x i ) ( i = 1 n x i p ) q K ( i = 1 n x i p q + n ) \large{\left(\prod_{i=1}^n x_i \right) \left(\sum_{i=1}^n x_i^p \right)^q \leq K \left(\sum_{i=1}^n x_i^{pq+n} \right)}


The answer is 16.48.

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1 solution

Reynan Henry
Dec 26, 2016

with AM-GM inequality we get 3 ( a 2003 + b 2003 + c 2003 ) ( ( a 3 + b 3 + c 3 ) ( a 2000 + b 2000 + c 2000 ) 3 a b c ( a 2000 + b 2000 + c 2000 ) 3(a^{2003}+b^{2003}+c^{2003})((a^3+b^3+c^3)(a^{2000}+b^{2000}+c^{2000})\ge 3abc(a^{2000}+b^{2000}+c^{2000}) (prove : expand) then set a 125 = x , b 125 = y , c 125 = z a^{125}=x,b^{125}=y,c^{125}=z we want to find the best K K for K ( x 16 + y 16 + z 16 ) ( x + y + z ) 16 K(x^{16}+y^{16}+z^{16})\ge(x+y+z)^{16} . We use PM-AM x 16 + y 16 + z 16 3 16 x + y + z 3 \sqrt[16]{\frac{x^{16}+y^{16}+z^{16}}{3}}\ge \frac{x+y+z}{3} so K = 3 15 K=3^{15}

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