K
kingwater


cho a,b,c,d là các số dương .Đặt
$S= \dfrac{a}{a+b+c}+\dfrac{b}{b+c+d}+\dfrac{c}{c+d+a}+\dfrac{d}{d+a+b}$
$CMR 1< S < 2$
$S= \dfrac{a}{a+b+c}+\dfrac{b}{b+c+d}+\dfrac{c}{c+d+a}+\dfrac{d}{d+a+b}$
$CMR 1< S < 2$
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