`bb(b o t)`

`" Ta có "`

`x +y + z = xyz to x = xyz - y -z`

`@ ) 1 + x^2 `

`= 1 + x ( xyz - y -z) `

`= 1 + x^2yz - xy - xz`

`= xy ( xz -1) - (xz - 1)`

`= (xy - 1)(zx- 1)`

`bb(T ư ơ n g)` `bb(t ự ) ` Ta được `:`

`@ ) 1 + y^2 = (xy -1)(yz -1)`

`@ ) 1 +z^2 = (zx -1)(yz -1)`

`" Khi đó "`

`bb(VT) = x/((xy -1)(zx-1)) + (2y)/((xy -1)(yz -1)) + (3z)/((xz -1)(yz -1))`

`bb(VT) = (x(yz -1) + 2y(zx -1) +3z(xy -1))/((xy -1)(yz -1)(zx- 1))`

`bb(VT) = (xyz - z + 2xyz - 2y + 3xyz - 3z)/((xy -1)(yz -1)(zx- 1))`

`bb(VT) = (6xyz - z - 2y - 3z)/((xy -1)(yz -1)(zx- 1))`

`bb(VT) = (6(x +y+z) - x -2y -3z)/((xy -1)(yz-1)(zx -1))`

`bb(VT) = (5x + 4y +3z)/((xy -1)(yz -1)(zx- 1)`

`" Lại có "` `:`

`bb(VP) = (xyz(5x +4y +3z))/((x+y)(y+z)(z+x))`

`bb(VP)=(xyz(5x + 4y +3z))/((xyz - z)(xyz - x)(xyz - y))`

`bb(VP) = (xyz(5x + 4y +3z))/(xyz(xy-1)(yz-1)(zx-1))`

`bb(VP) = (5x + 4y + 3z)/((xy -1)(yz-1)(zx-1)) `

`to bb(VT ) = bb(VP) ( đpcm ) .`