Hi,

I am trying to prove following identity. I obviously can't see something here.

[tex]\sum_{k=1}^{n}(\frac{\prod_{1\leq r\leq n-1}(y_k-z_r)}{\prod_{1\leq r\leq n, r\neq k}(y_k-y_r)})=1[/tex]

Where [tex]y_1,y_2,...,y_n[/tex] are all different.

Any hints?

Regards,

Ognjen