import sympy

x = sympy.Symbol('x')
xn = sympy.symarray('x', 3)  # от x_0 до x_2
an = sympy.symarray('a', 3)  # от a_0 до a_2

F = an[0]*(x-xn[0])*(x-xn[1])*(x-xn[2])+an[1]*(x-xn[0])*(x-xn[1])*(x-xn[2])+an[2]*(x-xn[0])*(x-xn[1])*(x-xn[2])

print(F.as_poly())