Question

giúp tôi với mn ơi

Answer 1

Lời giải:

$x+y+z=0\Rightarrow x+y=-z$. Khi đó:

$x^2+y^2-z^2=(x+y)^2-2xy-z^2=(-z)^2-2xy-z^2=-2xy$

Tương tự: $y^2+z^2-x^2=-2yz, z^2+x^2-y^2=-2xz$

Khi đó:

$A=\frac{xy}{-2xy}+\frac{yz}{-2yz}+\frac{zx}{-2zx}=\frac{1}{-2}+\frac{1}{-2}+\frac{1}{-2}=\frac{-3}{2}$