problem with 'solve' function to solve a set of equations

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Albert
Albert on 5 Oct 2016
Commented: Star Strider on 5 Oct 2016
Hi, everyone Is there anybody knowing how to solve issue of using 'solve' in matlab? I always have trouble using 'solve' in matlab. Here is a set of equations I am trying to solve with function 'solve' in matlab, but anyhow I can't get the desired result. In another word, matlab doesn't provide complete answers. Following is part of my code:
syms theta_A theta_B
p=-987614208*theta_A^24*theta_B^9*(theta_A - 1)^6*(theta_B - 1)^11;
[theta_A,theta_B]=solve('diff(P,theta_A)==0','diff(P,theta_B)==0','theta_A','theta_B');
The result that matlab gives is [0, 0];
I simple don't understand how this come. Because obviously theta_A=1 and theta_B=1 would satisfy the equations. Also there are more answers to be found.
Please help me out.
Greatly appreciate it!

Accepted Answer

Star Strider
Star Strider on 5 Oct 2016
You didn’t say what MATLAB version you’re using. The single-quote syntax is no longer appropriate in R2016b.
Otherwise, know that MATLAB is case-sensitive, so p~=P. Changing that gives the sort of result you likely want:
syms theta_A theta_B
P=-987614208*theta_A^24*theta_B^9*(theta_A - 1)^6*(theta_B - 1)^11;
[theta_A,theta_B]=solve(diff(P,theta_A)==0,diff(P,theta_B)==0,[theta_A,theta_B]);
theta_A =
4/5
0
0
1
theta_B =
9/20
0
1
0
  2 Comments
Star Strider
Star Strider on 5 Oct 2016
My pleasure.
Definitely read the Release Notes! They tell you the version difference syntax and functions. (I’m still reading the ones for R2016b.)
If you have R2016a, the problem was most likely the single-quotes, and not putting the variables you want to solve for in vector format (square brackets). Those changes gave the correct answer.

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