Evaluating Differentials

Question

Okay so I have the following problem, I am running this code in Python 3.6:
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from sympy import Symbol

x= Symbol('x')
y= Symbol('y')

y= x**4 + 7*x**3 + 8
y=diff(y,x)

print(y)
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Now this code runs perfectly, and gives me the output:
4*x**3 + 21*x**2

now after this, if i want to calculate the derivative at some value, let's say x=2... how to do that?

I will be grateful if someone helps me out, without changing the method, that is using the sympy itself.

Answer 1

Hi Saurabh,

You should use the subs() method for that:

print(y.subs(x, 2))

>>>
116

Answer 2

it worked, I am really thankful :)

Answer 3

so I am in bit of a situation:
I have evaluated a function Ylm using sympy, it is a function of 'theta' and 'phi'. Now when I am trying to plot this in this way:

>>>plot3d(Ylm ,(theta,0,3.14),(phi,0,6.28))

It is plotting in Cartesian system; but obviously I want to plot in spherical polar coordinates; so can anybody help me to plot Ylm in spherical polar coordinates.