I have the following code
xt = -1:0.1:1;yt=-100-(2*(cos(xt)).^3)-(4*(cos(xt)).^2)-3*cos(xt);plot(xt,yt)
and I get the blue curve as below. Now, I want to add a tangent line which must pass through the origin (as the black line I added "by hand" in the figure below).
I was thinking to use the function gradient but I am not sure how to impose the condition to pass through the (0,0).
Could you help me?