S
sevenlegend


tìm min max của S. biết $a,b,c,d\ge 0, \ \ \ a+b+c+d=4$
$S=\dfrac{a}{1+b^2}+\dfrac{b}{1+c^2}+\dfrac{c}{1+d^2}+\dfrac{d}{1+a^2}$
$S=\dfrac{a}{1+b^2}+\dfrac{b}{1+c^2}+\dfrac{c}{1+d^2}+\dfrac{d}{1+a^2}$
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