# derivative of a function of two variables

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Rasto on 21 Feb 2013
Commented: onur karakurt on 17 Oct 2018
Hello,
How can I get such a derivative in Matlab-Symbolic Toolbox ?
Differentiation according to two variables q1 and q2.
y=L1*cos(q1)+L2*cos(q2)
y´=-L1*sin(q1)-L2*cos(q2)
There is only this alternative?
z´=diff(y,q1)+diff(y,q2)
Thank you very much
onur karakurt on 17 Oct 2018
Thanks, but this is symbolic derivative, Can you give example numerical derivative with the function of y=L1*cos(q1)+L2*cos(q2)

Kaijie Cui on 21 Feb 2013
To get a numerical difference (symmetric difference), you calculate (f(x+dx)-f(x-dx))/(2*dx)
or "gradient", "polyder" (calculates the derivative of a polynomial) functions. Also a function "derivest" could also give numerical differentiation.

Babak on 21 Feb 2013
y = L1*cos(q1)+L2*cos(q2)
yp =-L1*sin(q1)-L2*cos(q2)
Do you want to take the derivative with respect to both independent parameters? if so, do this:
dy_over_dq1_dq2 = diff(diff(y,q1),q2)
Rasto on 22 Feb 2013
But if derivatives under him dy_over_dq1_dq2 = diff(diff(y,q1),q2) in Matlab me comes 0.