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subs in symbolic equation

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Saeed Heysiattalab
Saeed Heysiattalab 2021-5-21
回答： Hrishikesh Borate 2021-5-25
Hello.
I have calculated a partial derivative of a function as following:
clear
clc
syms x y r t real
f = @(r,t) r.*cos(t);
df_r = diff(f,r);
df_t = diff(f,t);
r = @(x,y) sqrt(x.^2 + y.^2);
dr_x = diff(r,x);
t = @(x,y) atan(y,x);
dt_x = diff(t,x);
df_x = df_r*dr_x + df_t*dt_x
the result is :
df_x =
(x*cos(t))/(x^2 + y^2)^(1/2) + (r*y*sin(t))/(x^2 + y^2)
How can I get the df_x in only r and t. I would like to substititute x^2+y^2 with r^2, x with r*cos(t) and y with r*sin(t).
Thanks

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Hrishikesh Borate
Hrishikesh Borate 2021-5-25
Hi,
It’s my understanding that you are trying to obtain df_x in terms of r and t. Following addition to the existing code will do the same:-
syms r t
df_x = subs(df_x, x.^2+y.^2, r.^2);
df_x = subs(df_x, [x,y], [r*cos(t),r*sin(t)]);
For more information refer to subs.

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