from math import*
from sympy import*

def Flagrange(X,Y) :
	n=len(X)
	x=symbols("x")
	P=0.
	for i in range(0,n) :
		L=1
		for j in range(0,n) :
			if i==j : continue
			L=L*(x-X[j])/(X[i]-X[j])
		P=P+L*Y[i]
	return simplify(P) 

# polynome dont la courbe passe par les points A(1,6), B(-3,-8) et C(8,-4)
X=[1,-3,8]
Y=[6,-8,-4] 
print(Flagrange(X,Y))