[Toán 7] Cần xin đề.

H

hiensau99

bài 1 đơn giản rồi nhé ;)

Bài 2:

Ta có:

$\frac{x}{x+y+z +t}+\frac{y}{x+y+t+z}+\frac{z}{y+z+t +x}+\frac{t}{x+z+t+y}< \frac{x}{x+y+z }+\frac{y}{x+y+t}+\frac{z}{y+z+t }+\frac{t}{x+z+t}< \frac{x}{x+y }+\frac{y}{x+y}+\frac{z}{z+t }+\frac{t}{z+t}$

$\Longrightarrow \frac{x+y+z+t}{x+y+z +t}< \frac{x}{x+y+z }+\frac{y}{x+y+t}+\frac{z}{y+z+t }+\frac{t}{x+z+t}< \frac{x+y}{x+y }+\frac{z+t}{z+t }$

$\Longrightarrow 1< \frac{x}{x+y+z }+\frac{y}{x+y+t}+\frac{z}{y+z+t }+\frac{t}{x+z+t}<2$

$\Longrightarrow$ M ko phải STN (đpcm)
 
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