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Algebra Difficulty 5.3 AIME, harder Prove it Ukraine

For positive aa, bb, cc, that satisfy the condition ab+bc+ca=3ab + bc + ca = 3, prove an inequality:
12a3+1+12b3+1+12c3+11. \frac{1}{2a^3+1} + \frac{1}{2b^3+1} + \frac{1}{2c^3+1} \ge 1.

Solution

Let us make such transformation:
1=ab+bc+ca3(abc)23abca1bc, b1ac, c1ab, 1 = \frac{ab+bc+ca}{3} \ge \sqrt[3]{(abc)^2} \Leftrightarrow abc \le a \le \frac{1}{bc},\ b \le \frac{1}{ac},\ c \le \frac{1}{ab},
hence a+b+c1ab+1bc+1caa+b+c \le \frac{1}{ab} + \frac{1}{bc} + \frac{1}{ca}. Then we use well-known inequality:
a12b1+a22b2++an2bn(a1+a2++an)2b1+b2++bn. \frac{a_1^2}{b_1} + \frac{a_2^2}{b_2} + \dots + \frac{a_n^2}{b_n} \ge \frac{(a_1+a_2+\dots+a_n)^2}{b_1+b_2+\dots+b_n}.
Then we will make such transformation:
12a3+1+12b3+1+12c3+1=1a22a+1a2+1b22b+1b2+1c22c+1c2(1a+1b+1c)22a+2b+2c+1a2+1b2+1c2(1a+1b+1c)221bc+1ca+1ab+1a2+1b2+1c2=(1a+1b+1c)2(1a+1b+1c)2=1, \begin{align*} \frac{1}{2a^3+1} + \frac{1}{2b^3+1} + \frac{1}{2c^3+1} &= \frac{\frac{1}{a^2}}{2a+\frac{1}{a^2}} + \frac{\frac{1}{b^2}}{2b+\frac{1}{b^2}} + \frac{\frac{1}{c^2}}{2c+\frac{1}{c^2}} \\ &\ge \frac{(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})^2}{2a+2b+2c+\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}} \\ &\ge \frac{\frac{(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})^2}{2}}{\frac{1}{bc}+\frac{1}{ca}+\frac{1}{ab}+\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}} \\ &= \frac{(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})^2}{(\frac{1}{a}+\frac{1}{b}+\frac{1}{c})^2} = 1, \end{align*}
That is what we had to prove.

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