How can I substitute a function into another equation to plot the answer?

Question

Mikhos 2012-11-29

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/55211-how-can-i-substitute-a-function-into-another-equation-to-plot-the-answer

评论： Rong 2023-10-29

采纳的回答： Matt Fig

在 MATLAB Online 中打开

I have an equation: r =1/16( a^4 + a^8/f(a) + f(a) )

of which f(a) = (2048 + 128a^6 + a^12 + sqrt(16384 + 2048a^6 + 80^12 + a^18))^1/3

I need to use MatLab to sub f(a) into r and thus plot r (and then continue to do more such substitutions in my model) however we havent been taught very much of Matlab and the more examples i find the mos confused and frustrated i get when things dont work.

I managed to get the following code so far and it brings up the error message below that. I think it might just be syntax or a command I dont know but appart from that i am stuck.

please could someone help me in this respect?

Code

a= sym('a')

r = sym('1/16.*(a.^4 + a.^8./F + F')

subs(r,'F','(2048 + 128.*a.^6 + a.^12 + sqrt(16384 + 2048.*a.^6 + 80.*.^12 + a.^18)).^1/3')

answer shown:

a =

a

??? Error using ==> sym.sym>convertExpression at 2547

Error: 'expression' expected [line 1, col 6]

Error in ==> sym.sym>convertChar at 2458

s = convertExpression(x);

Error in ==> sym.sym>convertCharWithOption at 2441

s = convertChar(x);

Error in ==> sym.sym>tomupad at 2195

S = convertCharWithOption(x,a);

Error in ==> sym.sym>sym.sym at 111 S.s = tomupad(x,'');

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Rong 2023-10-29

Update: in later versions, plot functions are separated into plot and fplot. Use fplot to plot a function, for instance:

syms x

p=@(x) ((1000/9*exp(x))/(exp(x)/9+1))

fplot( p(x))

if plot is instead used, the following error occurs:

Data must be numeric, datetime, duration, categorical, or an array convertible to double.

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Answer 1

Matt Fig 2012-11-29

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/55211-how-can-i-substitute-a-function-into-another-equation-to-plot-the-answer#answer_66991

在 MATLAB Online 中打开

f = @(a) (2048 + 128*a.^6 + a.^12 + sqrt(16384 + 2048*a.^6 + 80^12 + a.^18)).^(1/3);
r =@(a) (1/16)*( a.^4 + a.^8./f(a) + f(a) );
x = 0:.01:5;
plot(x,r(x))