MATLAB Function Finds The Derivative and Evaluates at Point A

My task is to write a MATLAB function that calculates the derivate of a polynomial, represented in vector form, and then also evaluates the derivative at a point a. So far I have the code to find the derivative, I just do not know how to evaluate at a from there. Note: f(x) = 3x^4 - 5x^2 -6x can be represented as P = [3, 0, -5, -6, 0]. Here is what I have so far:

function [PP,PPa] = ex5(P,a)

%

% where PP = the derivative of P

% PPa = the derivative of P evaluated at a

n = length(P);

PP = (n-1:-1:1).*P(1:n-1);

PPa = %not sure what to put here

[Jon]

PPa = polyval(PP,a)

[Jon]

By the way, if it is helpful there is also a MATLAB function polyder that will compute your PP

[Sophie Culhane]

I am not able to use any of the specialized MATLAB commands such as “polyval” or “polyder”. Is there any other alternative to these commands I could try?

[Steven Lord]

If I gave you a polynomial like and asked you to evaluate it (by hand, with pencil and paper) at x = 4 how would you do that? List the steps, not just the answer. Can you adapt your pencil and paper technique into MATLAB code and use it to evaluate p = [1 2 3] at x = 4;?

Use polyval not as part of your implementation but to check your answer during development, comparing the result of your code to the results from polyval.

Answer 1

[James Tursa]

@Sophie: You might investigate the .^ operator (element-wise raise-to-power).

[Sophie Culhane]

I figured it out using the .^ operator. Thank you.