Problem 22 Let a,b,c be positive real numbers such that a+b+c=3. Prove that a2b+b2c+c2a+2(ab2+bc2+ca2)≤63
Solution
Solution: The inequality equivalent to ⇔2∑cyca2b+4∑cycab2≤123⇔3∑syma2(b+c)+(∑cycab2−∑cyca2b)≤123⇔3∑cyca2(b+c)+(a−b)(b−c)(c−a)≤123
We only need to prove the inequality in the case (a−b)(b−c)(c−a)≥0. 3∑sym a2(b+c)+(a−b)2(b−c)2(c−a)2≤123⇔3(pq−3r)+p2q2+18pqr−27r2−4q3−4p3r≤123⇔p2q2+18pqr−27r2−4q3−4p3r≤(123−3pq+9r)2⇔f(r)=108r2+(4p3−72pq+2163)r+4q3+8p2q2−723pq+432≥0
Putting rct=108216q−108−2163 Let two cases 1) 0≤q≤2162163+108⇒rct≤0 f(0)=4(q+12+63)(q+3−3)2≥0 2) 2162163+108≤q≤3⇒rct≥0 f(rct)=4q3−36q2+108q+81−1083≥0,∀q∈[3+21;3]
So f(r)≥r∀r≥0. The solution is end.
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