Langrage`s Interpolating Formula Derivation

Here we are interpolating a polynomial y = f(x). Where (x0,y0), (x1,y2), (x3,y3),……. (xn,yn) are n+1 calculated point. Let their points are exist
Xi= x0+ih where i =1, 2, 3….n
Let
y = f(x) = a0(x-x1)(x-x2)(x-x3)……(x-xn)