[Toán 7] Tìm ẩn số chưa biết

P

pinkylun

Ta có:

$0 \le a \le b \le c \le 1$

$<=>(a-1)(b-1) \ge 0 $

$<=>ab-a-b+1 \ge 0$

$<=>ab+1 \ge a+b$

$<=>\dfrac{c}{ab+1} \le \dfrac{c}{a+b}$ :|

Tương tự như trên

$=>\dfrac{a}{bc+1} \le \dfrac{a}{b+c}$ :)

$=>\dfrac{b}{ac+1} \le \dfrac{b}{a+c}$ :(

:) :| :( $=>\dfrac{a}{bc+1}+\dfrac{b}{ac+1}+\dfrac{c}{ac+1} \le \dfrac{2a+2b+2c}{a+b+c}=2=>đpcm $ :D
 
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