# How to solve for x in an sin equation?

9 views (last 30 days)
aw007 on 19 Sep 2020
Commented: aw007 on 19 Sep 2020
Hi MATLAB wizards
I am currently trying to solve the following equation; -7sin(x/7)=0 using symbolic algebra and the solve function in MATLAB.
The code that I used is as follows:
syms x
solve( (-7*sin(x/7)),x)
This code returns the value 0, which is one of the solutions, the other one is 7*pi.
So how can I produce both of these solutions?
Any form of help would be appreciated.

John D'Errico on 19 Sep 2020
Edited: John D'Errico on 19 Sep 2020
No. There are INFINITELY many solutions. So you cannot say you want the OTHER solution. You can simply enough find a positive solution though.
syms x positive
solve( (-7*sin(x/7)),x)
ans =
7*pi

#### 1 Comment

aw007 on 19 Sep 2020
Yes you are right there are many solutions.
But this seems to do the trick.
Cheers for the help.

David Hill on 19 Sep 2020
[solx,parameters,conditions] = solve(eqn,x,'ReturnConditions',true);

#### 1 Comment

aw007 on 19 Sep 2020
yeah that seems to work @David Hill.
But the output is like this:
solx =
7*pi*k
parameters =
k
conditions =
in(k, 'integer')
Not sure if I understand the k and conditions bits.
Here's the code I used in conjunction with yours:
eqn=-7*sin(x/7);
[solx,parameters,conditions] = solve(eqn,x,'ReturnConditions',true)

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