AlgebraDifficulty 7.8National Olympiad, round 2Prove itUnited States
Let a, b, c be real numbers in the interval [0,1] with a+b, b+c, c+a≥1. Prove that 1≤(1−a)2+(1−b)2+(1−c)2+a2+b2+c222abc.
Solution
We may assume without loss of generality that a≥b≥c. Noting that (1−b)2+(1−c)2=(b+c−1)2−2bc+1, we may transform the right hand side of the desired inequality into (1−a)2+(1−b)2+(1−c)2+a2+b2+c222abc=a2+b2+c222abc−2bc+(1−a)2+(b+c−1)2+1. Rearranging, it suffices to show that a2+b2+c22bc(a2+b2+c2−2a)≤(1−a)2+(b+c−1)2,(19) which after rationalizing the numerator is equivalent to a2+b2+c2(2a+a2+b2+c2)2bc(b2+c2−a2)≤(1−a)2+(b+c−1)2.(20) If b2+c2−a2≤0, then (20) is clearly true. We may therefore assume that b2+c2−a2>0. To simplify (20), we want to produce the term b2+c2−a2 from the right-hand side of (20). By the RMS-AM inequality, we have (1−a)2+(b+c−1)2≥2[(1−a)+(b+c−1)]2=2(b+c−a)2.(21) By our assumption that a≥b≥c, we obtain b2+c2−a2−(b+c−a)2=2(ab+ac−bc−a2)=−2(a−b)(a−c)≤0, hence 2(b+c−a)2≥2b2+c2−a2≥0.(22) Together, (21) and (22) show that (1−a)2+(b+c−1)2≥2b2+c2−a2.(23) On the other hand, the ordering a≥b≥c≥0 implies a2≥bc, so a≥bc. Hence by the AM-GM inequality, we have a2+b2+c2(2a+a2+b2+c2)≥bc+2bc(2bc+bc+2bc)=(3+6)bc>4bc.(24) Dividing both sides of (24) by the left hand side and combining it with (23), we find that (1−a)2+(b+c−1)2≥2b2+c2−a2⋅a2+b2+c2(2a+a2+b2+c2)4bc, which is the desired (20).
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