ΒΆ  2018 USAMO Day 1 Problems and Solutions 
Individual Problems and Solutions 
For problems and detailed solutions to each of the 2018 USAMO Day 1  problems, please refer below:
 
Problem 1:  Let a , b , c  a, b, ca , b , c   be positive real numbers such that a + b + c = 4 a b c 3  a+b+c=4 \sqrt[3]{a b c}a + b + c = 4 3 a b c β  . Prove that
2 ( a b + b c + c a ) + 4 min β‘ ( a 2 , b 2 , c 2 ) β₯ a 2 + b 2 + c 2  2(a b+b c+c a)+4 \min \left(a^{2}, b^{2}, c^{2}\right) \geq a^{2}+b^{2}+c^{2}
2 ( a b + b c + c a ) + 4 min ( a 2 , b 2 , c 2 ) β₯ a 2 + b 2 + c 2 
Solution: 
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Problem 2:  Find all functions f : ( 0 , β ) β ( 0 , β )  f:(0, \infty) \rightarrow(0, \infty)f : ( 0 , β ) β ( 0 , β )   such that
f ( x + 1 y ) + f ( y + 1 z ) + f ( z + 1 x ) = 1  f\left(x+\frac{1}{y}\right)+f\left(y+\frac{1}{z}\right)+f\left(z+\frac{1}{x}\right)=1
f ( x + y 1 β ) + f ( y + z 1 β ) + f ( z + x 1 β ) = 1 
for all x , y , z > 0  x, y, z>0x , y , z > 0   with x y z = 1  x y z=1x y z = 1  .
Solution: 
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Problem 3:  For a given integer n β₯ 2  n \geq 2n β₯ 2  , let { a 1 , a 2 , β¦ , a m }  \left\{a_{1}, a_{2}, \ldots, a_{m}\right\}{ a 1 β , a 2 β , β¦ , a m β }   be the set of positive integers less than n  nn   that are relatively prime to n  nn  . Prove that if every prime that divides m  mm   also divides n  nn  , then a 1 k + a 2 k + β― + a m k  a_{1}^{k}+a_{2}^{k}+\cdots+a_{m}^{k}a 1 k β + a 2 k β + β― + a m k β   is divisible by m  mm   for every positive integer k  kk  .
Solution: 
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The problems on this page are the property of the MAA's American Mathematics Competitions