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Wojtek
Posts:
8
Registered:
5/7/12


Calculating 1st derivative with a parameter
Posted:
May 6, 2012 10:21 AM


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.



