Maths Olympiad Prep

Library / /48 of 520

Algebra Difficulty 5.8 AIME, harder Prove it

Example 4: Prove that when k4k \geq 4, we have
xyz(y+z)3+kxyz38+k(x>0,y>0,z>0)\sum \frac{x y z}{(y+z)^{3}+k x y z} \leq \frac{3}{8+k}(x>0, y>0, z>0)

Solution

Prove that after rearranging the above equation, we get
(3.2.8)3f0.4(9)+12f0.5(9)+(8+2k)f1,1(9)+(20+8k)f1,2(9)+(1+14k)f1,3(9)+(56+34k)f1,4(9)+(48+28k+2k2)f2,1(9)+(8+8k+5k2)f2,2(9)\begin{aligned} (3.2 .8) \Longleftrightarrow & 3 f_{0.4}^{(9)}+12 f_{0.5}^{(9)}+(-8+2 k) f_{1,1}^{(9)}+(-20+8 k) f_{1,2}^{(9)}+(1+14 k) f_{1,3}^{(9)} \\ & +(56+34 k) f_{1,4}^{(9)}+\left(-48+28 k+2 k^{2}\right) f_{2,1}^{(9)}+\left(8+8 k+5 k^{2}\right) f_{2,2}^{(9)} \end{aligned}

When k4k \geq 4, we have
2k80,8k200,1+14k0,56+34k0,2k2+28k480,5k2+8k+802 k-8 \geq 0, \quad 8 k-20 \geq 0, \quad 1+14 k \geq 0, \quad 56+34 k \geq 0, \quad 2 k^{2}+28 k-48 \geq 0, \quad 5 k^{2}+8 k+8 \geq 0

Thus, we know that (3.2.8) holds. Proof completed.

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.