Đáp án + giải thích các bước giải:
`xy+yz+zx=1`
`->1+x^2=xy+yz+zx+x^2=y(x+z)+x(x+z)=(x+y)(x+z)`
`->1+y^2=xy+yz+zx+y^2=x(y+z)+y(z+y)=(x+y)(y+z)`
`->1+z^2=xy+yz+zx+z^2=x(y+z)+z(z+y)=(x+z)(z+y) `
`->P=x\sqrt{((x+y)(y+z)(x+z)(z+y))/((x+y)(x+z))}+y\sqrt{((x+y)(x+z)(x+z)(z+y))/((x+y)(y+z))}+z\sqrt{((x+y)(x+z)(x+y)(y+z))/((x+z)(z+y))}`
`=x\sqrt{(y+z)^2}+y\sqrt{(x+z)^2}+z\sqrt{(x+y)^2}`
`=x(y+z)+y(x+z)+z(x+y)`
`=xy+zx+yx+yz+zx+zy`
`=2(xy+yz+zx)`
`=2.1`
`=2`
