I want to find R, but it says I have error on using fzero

Question

kanghao he 2016-12-6

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评论： Roger Stafford 2016-12-6

a=2.1.*10.^-3;
b=2.9.*10.^-4;
c=8.7.*10.^-8;
T=322 %Kelvins
resistance=@(R)(T(a+b.*log(R)+c.*(log(R)).^3)); %ohm
solution=fzero(resistance,200)

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Roger Stafford 2016-12-6

You don’t need to use ‘fzero’ in this equation. First solve the cubic equation for log( R ) using ‘roots’, and then solve for R using the ‘exp’ function, assumed your logs are natural logs, or powers of ten assuming the logs are base ten.

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Answer 1

Steven Lord 2016-12-6

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在 MATLAB Online 中打开

Your function is:

resistance=@(R)(T(a+b.*log(R )+c.*(log(R )).^3));

What is element (a+b.*log(R )+c.*(log(R )).^3) of the array T? Does that even evaluate to an integer value suitable for use as an index into an array? [That seems highly unlikely given your values of a, b, and c.]

I think you forgot an arithmetic operation (I'm guessing a minus sign) in your function.

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Steven Lord 2016-12-6

No, you did post them. I recommend you go back and check that you've correctly written the function you intend to solve, because I'm pretty sure you didn't copy it into MATLAB correctly based on the plots I made.

Walter Roberson 2016-12-6

Steven is pointing out that you have T(expression) and that in MATLAB that is an indexing operation. Perhaps you meant T*(expression) or T-(expression)

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