## How to calculate error in Lagrange interpolation?

Hi everybody.
I have a problem with Lagrange interpolation.
I don't understand the formular of calculating error clearly.

For example. After i find the polynomial Ln(x) by applying Lagragne interpolation. I have to find its error.
With n space, i have to calculate the derivative (n + 1) of that polynomial

The formular of error in Lagrange interpolation is:
R1 = |f(n+1)(x) - Ln(x)| <= ( M * |(t-x0)(t-x1)...(t-xn)| ) / (n+1)!

f(n+1)(x)<=M.
with t = x but t is not equals xi (i = 0,1...n).
and t, xi are in [a,b].
So do i have to find specific value of t,M to find the specific value of error R!. How can i do that?