MATLAB: How to draw the tangent to a curve passing through the origin

Hi,
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?

• Assuming that you want to obtain tangent lines which pass through the (-110,0), how about the following?
In this code, I changed delta xt from 0.1 to 0.01 in order to obtaion more accurate result.
xt = -1:0.01:1;yt =-100-(2*(cos(xt)).^3)-(4*(cos(xt)).^2)-3*cos(xt);dyt = gradient(yt,0.01);% yt value of tangent line at xt = 0 for each point on the curveytValue = yt - xt.*dyt;% Find the xt positions where ytValue is close to -110[~,pt] = mink(abs(ytValue + 110),2);% Tangent lineytTangent1 = dyt(pt(1))*(xt - xt(pt(1))) + yt(pt(1));ytTangent2 = dyt(pt(2))*(xt - xt(pt(2))) + yt(pt(2));% Draw the curve and the tangent linefigureplot(xt,yt)hold onplot(xt,ytTangent1)plot(xt,ytTangent2)grid on