Problem: Let a, b, c be positive real numbers such that a2+b2+c2=48. Prove a22b3+16+b22c3+16+c22a3+16≤242 When does equality hold?
Solution
Solution: Observe that 2x2+16=2(x2+8)=2(x+2)(x3=2x+4). From AM-GM: 2x3+16=(2x+4)(x3−2x+4)≤22x+4+x3−2x+4=2x2+8 By adding the inequality (1) obtained for x=a, x=b and x=c it suffices to prove: a3b3+8a3+b3c3+8b3+c3c3+8c3≤2⋅243, Since a3b3+b3c3+c3a3≤3(a3+b3+c3)3, using a3+b2+c2=48, we get the stated inequality.
Equality holds only when a=b=c=4.
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