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Re: Calculating 1st derivative with a parameter
Posted:
May 6, 2012 12:19 PM


"Wojtek" wrote in message <jo61cj$m95$1@newscl01ah.mathworks.com>... > Hello, > in my program I want to calculate the 1st derivative of a function y (it's a function of 'x1'). Let's say I want to check if the 1st derivative of this function in some specific point is smaller then 0.05. I have: > > syms x1; > y = ( x1  za )*( x1  zb )*( x1  zc )*( x1  zd ); %my function, all the parameters like 'za' were previously declared > > dydx=diff(y,x1,1); > > x_val=1; > dydx_val=subs(dydx,x1,x_val); > k2 = solve(dydx_val); > dydx_val =double(k2); > > if (dydx_val<0.05) > remember=x_val; > end > > > unfortunately the derivative is not calculated. I get information: > Warning: 1 equations in 0 variables. > Warning: Explicit solution could not be found. > and when I try to display dydx_val i get: > > dydx_val = > > [] > > I tried different methods, none of them was good (although I could do something wrong). I will be grateful for suggestions how can I calculate 1st derivative in a specific point and have the value of this derivative in double.
Hello,
If I understood your question correctly, you don't need the line for solve and the following seems to work for me:
syms za zb zc zd x1; y = ( x1  za )*( x1  zb )*( x1  zc )*( x1  zd );
xVal = 1;
% Take the derivative and substitute the value of x1 focValue = subs(diff(y,'x1'),x1,xVal);
if focValue < 0.5 remember = focValue; end
I hope this helps,
A.



