def d2free(x,T): h = 0.00001 x1 = x-h x2 = x+h y = free(x,T) y1 = free(x1,T) y2 = free(x2,T) return (y2-2.0*y+y1)/h**2
def f2(x): return ((xc-x1)*free(x,T)+(x-xc)*free(x1,T))/(x-x1)
def f1(x): return ((xc-x)*free(x2,T)+(x2-xc)*free(x,T))/(x2-x)