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Algebra Difficulty 6.9 National olympiad Prove it

(2) Let a,b,ca, b, c be positive real numbers, and a2+b2+c2=1a^{2}+b^{2}+c^{2}=1, prove:
a5+b5ab(a+b)+b5+c5bc(b+c)+c5+a5ca(c+a)65(ab+bc+ca)\frac{a^{5}+b^{5}}{a b(a+b)}+\frac{b^{5}+c^{5}}{b c(b+c)}+\frac{c^{5}+a^{5}}{c a(c+a)} \geqslant 6-5(a b+b c+c a)

Solution

(2) a5+b5ab(a+b)=a4+b4ab(a2+b2)+a2b2a+b=\frac{a^{5}+b^{5}}{a b(a+b)}=\frac{a^{4}+b^{4}-a b\left(a^{2}+b^{2}\right)+a^{2} b^{2}}{a+b}=
(ab)4+4ab(a2+b2)6a2b2ab(a2+b2)+a2b2ab\frac{(a-b)^{4}+4 a b\left(a^{2}+b^{2}\right)-6 a^{2} b^{2}-a b\left(a^{2}+b^{2}\right)+a^{2} b^{2}}{a b} \geqslant
3ab(a2+b2)5a2b2ab=3(a2+b2)5ab\frac{3 a b\left(a^{2}+b^{2}\right)-5 a^{2} b^{2}}{a b}=3\left(a^{2}+b^{2}\right)-5 a b

Similarly, b5+c5bc(b+c)3(b2+c2)5bc,c5+a5ca(c+a)3(c2+a2)5ca\frac{b^{5}+c^{5}}{b c(b+c)} \geqslant 3\left(b^{2}+c^{2}\right)-5 b c, \frac{c^{5}+a^{5}}{c a(c+a)} \geqslant 3\left(c^{2}+a^{2}\right)-5 c a, adding them up we get
a5+b5ab(a+b)+b5+c5bc(b+c)+c5+a5ca(c+a)65(ab+bc+ca)\frac{a^{5}+b^{5}}{a b(a+b)}+\frac{b^{5}+c^{5}}{b c(b+c)}+\frac{c^{5}+a^{5}}{c a(c+a)} \geqslant 6-5(a b+b c+c a)

Since a2+b2+c2=1ab+bc+caa^{2}+b^{2}+c^{2}=1 \geqslant a b+b c+c a, we have 65(ab+bc+ca)3(ab+6-5(a b+b c+c a) \geqslant 3(a b+ bc+ca)2b c+c a)-2. Therefore, (2) is a generalization of (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.