Ta có: `x+y+z=0`
`=>x+y=-z`
`A=\frac{x^2}{yz}+\frac{y^2}{zx}+\frac{z^2}{xy}(x,y,z\ne0)`
`=\frac{x^3}{xyz}+\frac{y^3}{xyz}+\frac{z^3}{xyz}`
`=\frac{x^3+y^3+z^3}{xyz}`
`=\frac{(x+y)(x^2-xy+y^2)+z^3}{xyz}`
`=\frac{-z(x^2-xy+y^2)+z^3}{xyz}`
`=\frac{-z(x^2-xy+y^2-z^2)}{xyz}`
`=\frac{-z(x^2+2xy+y^2-3xy-z^2)}{xyz}`
`=\frac{-[(x+y)^2-z^2-3xy]}{xy}`
`=\frac{-[(-z)^2-z^2-3xy]}{xy}`
`=\frac{-(z^2-z^2-3xy)}{xy}`
`=\frac{-(-3xy)}{xy}`
`=\frac{3xy}{xy}`
`=3`
`->A=3`
