a/(a+b+c) ≤ a/(a+b) car c >0
b/(b+c +a) ≤ b/(b+c) car a>0
c/(c+a+b) ≤ c/(c+a) car b >0
donc
(a+b+c)/(a+b+c) ≤ a/(a+b) + b/(b+c) + c/(c+a)
1 ≤a/(a+b) + b/(b+c) + c/(c+a)
a/(a+b) +b/(b+c) + c/(c+a) -2 =
( a(b+c)(c+a) + b(a+c)(a+b) +c(a+b)(b+c) ) /(a+b)(b+c)(c+a) -2
( abc +a²b+ac²+a²c+ba²+b²a+abc+b²c+abc+c²a+cb²+c²b)/(a+b)(b+c)(c+a) -2
(3abc + 2a²b + 2ac² + a²c + ab² + 2b²c +c²b) /(a+b)(b+c)(c+a) - 2 =
((3abc +2a²b+2ac² +a²c+ab²+2b²c+c²b - 2(abc +a²b+ac²+a²c+b²c+b²a+bc²+bca)) / (a+b)(b+c)(c+a) =
( -abc - a²c -ab² - c²b ) / (a+b)(b+c)(c+a) ≤ 0
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a/(a+b+c) ≤ a/(a+b) car c >0
b/(b+c +a) ≤ b/(b+c) car a>0
c/(c+a+b) ≤ c/(c+a) car b >0
donc
(a+b+c)/(a+b+c) ≤ a/(a+b) + b/(b+c) + c/(c+a)
1 ≤a/(a+b) + b/(b+c) + c/(c+a)
a/(a+b) +b/(b+c) + c/(c+a) -2 =
( a(b+c)(c+a) + b(a+c)(a+b) +c(a+b)(b+c) ) /(a+b)(b+c)(c+a) -2
( abc +a²b+ac²+a²c+ba²+b²a+abc+b²c+abc+c²a+cb²+c²b)/(a+b)(b+c)(c+a) -2
(3abc + 2a²b + 2ac² + a²c + ab² + 2b²c +c²b) /(a+b)(b+c)(c+a) - 2 =
((3abc +2a²b+2ac² +a²c+ab²+2b²c+c²b - 2(abc +a²b+ac²+a²c+b²c+b²a+bc²+bca)) / (a+b)(b+c)(c+a) =
( -abc - a²c -ab² - c²b ) / (a+b)(b+c)(c+a) ≤ 0