CODEWITHSUNDEEP
pythonsympy
Salil S. Kulkarni
Salil S. Kulkarni

Reputation: 23

Extract the coefficient of trignometric expression

I have tried extracting coefficients of a trignometric expression using multiple methods (symp.coeff, as_coefficient). The code is as follows:

import sympy as sy

x1 = sy.symbols('x1')
A, B = sy.symbols('A,B')
L = sy.symbols('L', positive=True)

f = (A * sy.sin(sy.pi * x1 / (2 * L)) + B * sy.cos(sy.pi * x1 / (2 * L))) / L ** 2

c1 = f.coeff(sy.cos(sy.pi * x1 / (2 * L)))
# or
c1 = f.as_coefficient(sy.cos(sy.pi * x1 / (2 * L)))

display(c1)

The output that is expected is B/L**2. But in the first case, I get the output to be 0 while in the second case I get to be "None". Can someone please guide me in how to extract the coefficient?

Upvotes: 0

Views: 37

Answers (1)

hpaulj
hpaulj

Reputation: 231385

I get what you want if I expand the expression, so it's a simple sum, not a product:

In [3]: f
Out[3]: 
     ⎛π⋅x₁⎞        ⎛π⋅x₁⎞
A⋅sin⎜────⎟ + B⋅cos⎜────⎟
     ⎝2⋅L ⎠        ⎝2⋅L ⎠
─────────────────────────
             2           
            L   

In [6]: f.expand()
Out[6]: 
     ⎛π⋅x₁⎞        ⎛π⋅x₁⎞
A⋅sin⎜────⎟   B⋅cos⎜────⎟
     ⎝2⋅L ⎠        ⎝2⋅L ⎠
─────────── + ───────────
      2             2    
     L             L     

In [7]: f1 = f.expand()

In [8]: f1.coeff(cos(pi*x1/(2*L)))
Out[8]: 
B 
──
 2
L

compare the types:

In [9]: type(f)
Out[9]: sympy.core.mul.Mul

In [10]: type(f1)
Out[10]: sympy.core.add.Add

Upvotes: 1

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