Let a, b and c be positive real numbers such that abc=1. Prove that the following inequality holds 21(a+b+c)+1+a1+1+b1+1+c1≥3 When does equality hold?
Solution
Since (1−bc)2≥0 it follows that 1+bc≥2bc, i.e. 2bc1≥1+bc1. We get that 2a+1+a1=2bc1+1+a1≥1+bc1+1+a1=1+a11+1+a1=1…(1). In the same way we prove that 2b+1+b1≥1…(2) and 2c+1+c1≥1…(3).
By adding (1), (2) and (3) we get the required inequality. Let us note that equality holds if and only if 1=bc, i.e. a=1. In the same way we get b=1 and c=1.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement and solution reproduced as published; topic and difficulty added by this site.