Multiple answers from solve

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Kevin
Kevin on 5 Sep 2021
Commented: Kevin on 6 Sep 2021
close all
clear
clc
AB=180
AO=60
BO=200
syms alpha
eqn1 = BO^2 == AB^2+AO^2-2*AB*AO*cosd(alpha)
eqn2 = solve (eqn1,alpha)
vpa(eqn2)
ans =
259.32807071423785412135760253313
100.67192928576214587864239746687
I know that it's the latter (100.6719....), but why does it give me 2 answers and how do I make matlab give me 1 answer?

Accepted Answer

Walter Roberson
Walter Roberson on 5 Sep 2021
AB=180
AB = 180
AO=60
AO = 60
BO=200
BO = 200
syms alpha
eqn1 = BO^2 == AB^2+AO^2-2*AB*AO*cosd(alpha)
eqn1 = 
sol = solve(eqn1, alpha)
sol = 
vpa(sol)
ans = 
How is MATLAB to know that one of the solutions is more right than the other?
You could vpasolve() with a range:
vpasolve(eqn1, alpha, [0 180])
ans = 
100.67192928576214587864239746687
  3 Comments
Kevin
Kevin on 6 Sep 2021
Thank you so much.

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