# Find Coefficient of a Trigonometric Function

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ercan duzgun on 3 Jan 2022
Commented: ercan duzgun on 7 Jan 2022
I would like to find the coeffienct of the trigonometric function by separated terms. How can I do that?
My codes are :
clear all;close all;clc;
syms h h1 h2 phi12 phi34 l1 l2 l3 l4 l5 l6 a b
x1=h/3-h1*cos(phi12)
y1=(l1^2-l2^2)/(2*a)
z1=h1*sin(phi12)
x2=(-1/2)*(h/3-h2*cos(phi34))-sqrt(3)/2*((l3^2-l4^2)/(2*a))
y2=(sqrt(3)/2)*(h/3-h2*cos(phi34))-1/2*((l3^2-l4^2)/(2*a))
z2=h2*sin(phi34)
P1P2=expand((x1-x2)^2+(y1-y2)^2+(z1-z2)^2-b^2)
[P1P2_c_cosphi34,P1P2_t_cos34]=coeffs(P1P2,cos(phi34)); C21=P1P2_c_cosphi34(2)
[P1P2_c_cosphi12,P1P2_t_cos12]=coeffs(P1P2,cos(phi12)); D21=P1P2_c_cosphi12(2)
I want to make the equation in this form of  Here I want to find the coefficient of A21, B21, C21, D21 and E21. How can I do this symbolically?

VIGNESH B S on 7 Jan 2022
syms x
f = 5.*x.^2 + 7.*x + 3
coeffs(f)
Use coeffs function.
Returns x^0 , x^1 .. ,x^n coeffs.
Also works with sin(x) , cos(x)..
ercan duzgun on 7 Jan 2022
I used that function of coeffs. It is in my code , as you can see. However it only find some of the coefficients in my program. I can only find C21 and D21 coefficient. I can't find other coefficient of A21 B21 E21.

R2019b

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