Giải: $b+\sqrt[4]{\dfrac{1}{3125}}+\sqrt[4]{\dfrac{1}{3125}}+\sqrt[4]{\dfrac{1}{3125}}+\sqrt[4]{\dfrac{1}{3125}} \ge \sqrt[5]{b}$
$c+\sqrt[8]{\dfrac{1}{387420489}}+...+\sqrt[8]{\dfrac{1}{387420489}} \ge \sqrt[9]{c}$ ($8$ phần tử $\sqrt[8]{\dfrac{1}{387420489}}$)
Suy ra $A \le 1+4\sqrt[4]{\dfrac{1}{3125}}+8\sqrt[8]{\dfrac{1}{38420489}}$
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