MATLAB Answers


Need to solve this

Asked by Abdullah Azzam on 1 Jun 2019
Latest activity Commented on by Walter Roberson
on 12 Jul 2019
Hi guys I need away to solve these 3 equations and get the value of a b and c
a+b = 204
a+b*exp(-105*c) = 192.6
a+b*exp(-180*c) = 177.6
I have tried to use vpasolve but it gives wrong answer. What other way to use?


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2 Answers

Answer by Stephan
on 1 Jun 2019

I assume you want real Solutions. Then try:
syms a b c
assume ([a b c], 'real')
eq = [a+b == 204;...
a+b*exp(-105*c) == 192.6;...
a+b*exp(-180*c) == 177.6];
[a, b, c] = vpasolve(eq,[a b c])
Best regards


It gives wrong output it just make a=204 and b=0 and solve for c but when you substitute the result back in ghe equations you can varify they are wrong answers
Walter Roberson
on 12 Jul 2019
vpasolve() works to finite precision according to the digits() setting, and the calculations are suffering from massive loss of precision.
You can give a search range:
[A,B,C] = vpasolve(eq,[a b c],[200 230;-50 -1/2;-5 5])

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Answer by Alex Sha on 12 Jul 2019

Hi, How about the results belows:
a: 214.703486863113
b: -10.7034868631133
c: -0.00690634104047918


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