Is there a mistake in my code?
显示 更早的评论
Hello everyone,
I need to plot a graph and after many trials and errors I came up with this code:
syms A B
D1= 12*A + 19.7891*B == 78.8820;
D2= 19.7891*A + 449*B == 44700;
sol=solve([D1,D2], [A,B]);
A=sol.A % A= -169.9537
B=sol.B % B= 107.0451
a=exp(A)
x=2.5:0.1:10
y=a*(x.^B);
plot(x,y)
However the graph I end up with is nothing like the graph I need. This is the graph I get:
And this is the graph I need:
I double-checked the values of all the variables and they are all correct, so I'm thinking I must have made a mistake with the syntax or the commands themselves, but I can't figure it out. I would appreciate any help you can give me.
采纳的回答
I might be better to estimate the parameters directly —
x = [2.5 6 9]; % Excerpt From Plot
y = [1500 600 400]; % Excerpt From Plot
pwrfcn = @(b,x) b(1) .* x.^b(2); % Objective Function
[B,fv] = fminsearch(@(b)norm(y - pwrfcn(b,x)), rand(2,1))
xv = 2.5:0.1:10;
figure
plot(x, y, 'pk')
hold on
plot(xv, pwrfcn(B, xv), '-b')
hold off
grid
.
6 个评论
Arsel Tanriverdi
2022-1-10
编辑：Arsel Tanriverdi
2022-1-10
Hiro Yoshino
2022-1-10
[B,fv] = fminsearch(@(b)norm(y - pwrfcn(b,x)), rand(2,1))
fminsearch: rand(2,1) corresponds to the initial point.
Is there any reasons for using syms to solve the equation? It seems to be too much for what you are trying to do...
Arsel Tanriverdi
2022-1-10
D1= 12*A + 19.7891*B == 78.8820;
D2= 19.7891*A + 449*B == 44700;
becomes —
AB = [12 19.7891; 19.7891 449] \ [78.8820; 44700]
fprintf('A = %10.4f\nB = %10.4f', AB)
@Hiro — Thank you!
.
Arsel Tanriverdi
2022-1-10
Star Strider
2022-1-10
As always, (our) pleasure!
.
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