How to Draw a Tangent Line in Matlab

How I can find the slope of a tangent line and plot that tangent line in matlab

Discussion in 'MATLAB' started by Ion, Jan 22, 2009.

1. Hello every one.I have the follwing problem.
   I must write a function script to find the tangent line for the function f(x)=(A.*(x.^3))+B.*(x.^2)+(C.*x)+D.
   Executing the scriptfile must create a plot with f(x) including the tangentline at the specific point.

   Work only from the schedule below:

   •The syntax of the function definition line, must be as follows:

   function[slope]=tangentline(A,B,C,D, x, Location)

   • A,B,C,D : These are the coefficients of the polynomial function.
   • x :This is the name of the independent internal variable.x must be defined before calling this "tangentline"function. So don't use a number!.
   •Location :This variable tells you where the function must plot the tangentline. This location must be in the range of the independent variable.
   For x=3, we type only "3" instead of "Location"

   About the result, the output variable called "slope" will give you the slope of the tangent line at the located place.

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2. We won't do your homework for you, Ion. If you know how to dfferentiate polynomials and express the equation of a line knowing its slope and a point on it, then you should be able to solve the problem without any difficulty.

   Roger Stafford

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3. How about:
   Slope = 3*A*location^2+...;

   Not sure I understand the question. If you just want the slope at a single point it's this simple.

4. This is my function script.
   function [slop]=tangentline(A,B,C,D,x,Location)
   % A,B,C,D : These are the coefficients of the polynomial function.
   % x : This is the name of the independent internal
   % variable. x must be defined before calling this "tangentline"function. So don't use a number!.
   % Location : This variable tells you where the function must
   % plot the tangentline. This location must be in the
   % range of the independent variable.
   % For x=3, we type only "3" instead of "Location"

   slop=3*A.*Location+2*B.*Location+C

   y=x.^3+x.^2+x
   plot(x,y)
   hold on

   b=(A.*Location.^3)+(B.*Location.^2)+(C.*Location)+D
   y2=slop*(x-Location)+b
   plot(x,y2,'r')
   hold off

   But the problem apers when i call for this function
   For exampleMy slope it will be calculated but my plot is not shoing the real tangent line of that function.
   I wait for your answers.Thank you.

5. % I think you need to replace the line:
   slop=3*A.*Location+2*B.*Location+C
   % with:
   slop=3*A.*(Location^2)+2*B.*Location+C

   % See if that works.

6. I tried and it acts in the same way.The problem is still unsolved.

7. % OK, now you need to change:
   y=x.^3+x.^2+x
   % to
   y=A*x.^3+B*x.^2+C*x + D
   % to be consistent with b & slop

8. It dosen't change anything.My plot looks the same but i get a diffrent result for the slope.Thank you for your help.

9. % Then I guess I don't know what you are trying to do.
   % If I code this:

   % dummy data:
   A = 1;
   B = 2;
   C = 3;
   D = 4;
   Location = 5;
   x = linspace(3,9);
   % plot polynomial:
   y=A*x.^3+B*x.^2+C*x + D;
   plot(x,y)
   hold on
   % find slope at Location:
   slop=3*A.*(Location^2)+2*B.*Location+C;
   % find y at Location:
   b=(A.*Location.^3)+(B.*Location.^2)+(C.*Location)+D;
   % plot slope:
   y2=slop*(x-Location)+b;
   plot(x,y2,'r')
   hold off

   % I get the plot I thought you wanted.

10. Yes now it works. I don't know where it was my problem but thank you a lot.
    That's the plot i whant.Thank's again.

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How to Draw a Tangent Line in Matlab

Source: https://www.math-forums.com/threads/how-i-can-find-the-slope-of-a-tangent-line-and-plot-that-tangent-line-in-matlab.193115/

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