CM rằng: `(y-z)/( ( x-y) (x-z) ) = (z-x)/( ( y-z) (y-x) ) = (x-y)/( (z-x) (z-y) ) = 2/(x-y) + 2/(y-z) + 2/(z-x)`

ĐK : `x\ne y\ne z`

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

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

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

Cộng các vế :

`-> (y-z)/((x-y)(x-z)) + (z-x)/((y-z)(y-z)) + (x-y)/((z-x)(z-y))=1/(z-x)+1/(x-y)+1/(x-y) + 1/(y-z) + 1/(y-z) +1/(z-x)`

`-> (y-z)/((x-y)(x-z)) + (z-x)/((y-z)(y-z)) + (x-y)/((z-x)(z-y))=2/(x-y) +2/(y-z) +2/(z-x)`