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I am new to matlab and want to calculate something like f(x)/f'(x). I want the user to input the function f(x), the parameter x and a value of x (suppose 5,so that I can evaluate f(5)/f'(5)) .Please suggest what I should do.

Cœur
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Mathematica Noob
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  • Do you have symbolic toolbox? There are two very different approaches depending on whether or not you use it. – Ben Voigt Feb 12 '17 at 20:23
  • Be sure to let me know if the answer I have provided works for you. – codingEnthusiast Feb 12 '17 at 21:03
  • @ Ben Voigt :I searched around and found a method to input a function,its parameter and the value but NOT the differentiation. However, I would be grateful I you could tell me about the two approaches. – Mathematica Noob Feb 12 '17 at 21:04

2 Answers2

0

One approach is to use symbolic variables

function [ val ] = func( fun, num )
    symfun = sym(fun);
    dsymfun = diff(symfun);
    y = symfun/dsymfun;
    val = subs(y, num);
end

and then call it

e.g. 

value = func('x^2', 5)

value =

5/2

Otherwise, you can provide your input as a symbolic variable:

function [ val ] = func( fun, num )
    dfun = diff(fun);
    y = fun/dfun;
    val = subs(y, num);
end

and then write

syms x;
func(x^2, 5)
codingEnthusiast
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0

You can do this using polyder and polyval as follows:

f = input('Enter f(x): '); %e.g; Enter [1 3 4]  if f(x)= x^2 + 3*x + 4
df = polyder(f);           %f'(x)
x= input('Enter x:  ');    %Enter the value of 'x' e.g 5
fx_dfx= polyval(f,x)/ polyval(df,x)    %f(x)/f'(x)

If you have Symbolic Math Toolbox, you can also do this using:

syms x;                      %Creating a symbolic variable x
f = input('Enter f(x): ');   %Enter f(x) e.g: x^2 + 3*x + 4
f(x)= f;                     %Converting sym to symfun
df(x) = diff(f)              %f'(x)
x_val= input('Enter x:  ');  %Enter the value of 'x' e.g 5
fx_dfx = double(f(x_val)/df(x_val)) %f(x)/f'(x)
Sardar Usama
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