Let a,b,c≥2 be real numbers, satisfying the condition a+b+c=a1+b1+c1+8. Prove that we have the inequality 3(ab+bc+ca)≤81≤(a+b+c)2.
Solution
Putting x=a+b+c then we have a+b+c=a1+b1+c1+8≥a+b+c9+8⇒x≥x9+8. Direct manipulation leads to (x−9)(x+1)≥0 or x≥9. Moreover, a+b+c=a1+b1+c1+8<21+21+21+8=219. Thus 9≤x<219. Now, we rewrite the given condition as: 2x=a2+b2+c2+16⇔aa−2+bb−2+cc−2=19−2x. By Cauchy-Schwarz inequality, we have aa−2+bb−2+cc−2≥a(a−2)+b(b−2)+c(c−2)[(a−2)+(b−2)+(c−2)]2=a2+b2+c2−2x(x−6)2. Thus, using previous equation, we have 19−2x≥a2+b2+c2−2x(x−6)2⇒a2+b2+c2≥2x+19−2x(x−6)2. In the other hands, the required inequality is equivalent to 54≥2(ab+bc+ca)⇔54+(a2+b2+c2)≥x2. By previously proved inequality, it is sufficient to prove that 54+2x+19−2x(x−6)2≥x2. Simple calculation leads us to the equivalent inequality (x−9)(x2−2x−59)≥0, which is true due x2−2x−59=(x−1)2−60≥82−60>0. The equality holds in both sides if and only if a=b=c=3.
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