MATLAB Answers

## Need to solve this

Asked by Abdullah Azzam

### Abdullah Azzam (view profile)

on 1 Jun 2019
Latest activity Commented on by Walter Roberson

### Walter Roberson (view profile)

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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R2017a

## 2 Answers

### Stephan (view profile)

Answer by Stephan

### Stephan (view profile)

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
Stephan

Abdullah Azzam

### Abdullah Azzam (view profile)

on 1 Jun 2019
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

### Walter Roberson (view profile)

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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### Alex Sha (view profile)

Answer by Alex Sha

### Alex Sha (view profile)

on 12 Jul 2019

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

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