how to write a function in interval?

2014 10 21
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更新时间：2022 2 8
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hi,
can you please help me to write this function code in matlab?
thank you in advance

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José-Luis
José-Luis 2014-10-21
编辑：José-Luis 2014-10-21
What have you tried so far?
I would recommend you to read the "Getting started" part of the documentation.
You might want to look at the fzero() function.
doc fzero
doc plot
doc fplot
lina
lina 2014-10-21
I attached the question in jpg form can't write the code :'(

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Matt Tearle
Matt Tearle 2014-10-21

0 个投票

MATLAB Academy can help you learn the basics of MATLAB. Also the Getting Started section of the doc, as Jose-Luis suggested.
For this specific question, you'll need to:
  1. create a vector of x values from -2 to 5.
  2. create a corresponding vector of y values by evaluating the function.
  3. plot y as a function of x.
  4. maybe pretty up the graph a bit.
For 2 you'll need to know about array operations. For 4 you can use xlabel, ylabel, title, xlim, ylim, and/or axis.
As Jose-Luis mentioned, if you need the roots exactly, you can use fzero, but that seems a bit advanced for HW#1. Because the function is a polynomial, you could also use polyval (for part 2) and roots. But, again, that seems more than is necessary.

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lina
lina 2014-10-21
thank you Matt it works, but why i get error in mpower ? this is what I start with
X=[-2:5]
x=X'
the_function =((x^4)-(5.*x^3)+(2.*x^2)+(6.*x)-(4));
Matt Tearle
Matt Tearle 2014-10-21
Array operations! You've done 5.*x, which is fine, but actually not needed because 5 is a scalar (so 5*x is the same as 5.*x). However, x is a vector, which means x^2 = x*x is a problem. That exponentiation is interpreted in a matrix sense, so that's where you need your array operator ( .* or .^).
[And because I can't help myself, the square brackets around -2:5 aren't necessary. The brackets are for concatenating values into an array, but the : already makes an array. x = (-2:5)'; will get you the vector a little quicker and cleaner.]
lina
lina 2014-10-21
Thank u very much u r awesome! It works
lina
lina 2014-10-21
pardon Matt but there is another problem in plotting dimension
x=(-2:5);
y=x.^4-5.*(x.^3)+2.*(x.^2)+6.*(x)-4;
plot(y,[-2,5]);
I get this error ??? Error using ==> plot Vectors must be the same lengths.
what should I do?
thank u in advance
Matt Tearle
Matt Tearle 2014-10-22
You don't need to specify the domain to the plot function, just the x values and their corresponding y values (in that order): plot(x,y)
If you want to adjust the axis limits that MATLAB chooses, use the xlim and/or ylim functions (use the doc to see how they work).

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