Let satisfy , find the maximum value of the expression above.
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Call the expression E, we rewrite it like this: E = c y c ∑ a , b , c a + a 2 1 . By AM-GM E ≤ c y c ∑ a , b , c 2 + a 1 1 Set x = a 1 , y = b 1 , z = c 1 then x y z = 1 and the R H S becomes c y c ∑ x , y , z 2 + x 1 . Now we consider 3 − 2 R H S = c y c ∑ x , y , z 2 + x x By Titu's Lemma 3 − 2 R H S ≥ x + y + z + 6 ( x + y + z ) 2 We see that x y + y z + z x ≥ A M − G M 3 3 x y z = 3 ⇒ x + y + z + 2 ( x y + y z + z x ) = ( x + y + z ) 2 ≥ x + y + z + 6 ∴ 3 − 2 R H S ≥ 1 ⇒ R H S ≤ 1 ∴ E ≤ 1 The equality holds when a = b = c = 1
Note : There's a much shorter solution using U.C.T