Solving an equation with log
显示 更早的评论
Hi i'm fairly new to MATLAB and encounter a problem regarding this equation y=c(x)^m
where m is the gradient of points:
(x,1y1)=(100,50)
(x2,y2)=(1000,10)
This is the eq i put on MATLAB:
Eq = log10(Y2) == log10(C1*X2^(m)); %Equation
C1 = vpasolve (Eq, C1)
It seems that i get C far from my hand-drawn answer
How to solve C?
采纳的回答
Looks okay to me.
format long g
X1 = 100; Y1 = 50;
X2 = 1000; Y2 = 10;
m = (Y2-Y1)./(X2-X1);
syms C1
Eq = log10(Y2) == log10(C1*X2^(m)); %Equation
C1sol = solve(Eq,C1)
vpa(C1sol)
%log10(Y2) == log10(C1*X2^m) implies
%Y2 == C1*X2^m implies
C1_numeric = Y2/(X2^m)
4 个评论
Rafi B
2021-3-20
Hi Walter, thank you so much for the feedback.
The answer for C given by the book is still way off, I'm not sure if it is wrong
or i misinterpret the question.
Any thoughts?
Walter Roberson
2021-3-20
syms x1 x2 y1 y2
syms c m
eqn1 = y1 == c*x1^m
eqn2 = y2 == c*x2^m
eqn3 = y1/y2 == (c*x1^m)/(c*x2^m)
You should be able to proceed from here, as the above eliminates one of the variables, so re-arrange to solve the other variable.
Rafi B
2021-3-20
Walter Roberson
2021-3-20
Yes, giving you an equation of the form A=B^m with known A and B, which you can use to find m easily.
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