from sympy import *
init_printing(use_unicode=True)
import matplotlib.pyplot as plt
from sympy.abc import x
from sympy.utilities.lambdify import lambdify, implemented_function
from sympy import Function


from sympy import *
x1, x2, z, t = symbols('x1 x2 z t')
f, g, h = symbols('f g h', cls=Function)
c1=1
c2=1
f=c1*x1**2+c2*x2**2
df1=diff(f,x1)
df2=diff(f,x2)


df1.evalf(subs={x1:6, x2:5})