asin and acos quadrant check function help

Hi I'm trying to work out how to write a code for a quadrant check to get the right angle. An example of what I'm trying to acheive is below:

The C11, C21, C31 are matrix elements from a direciton cosine matrix

Alpha=78.52

Sigma1=asin(C21./sind(Alpha)); % where C21./sind(Alpha)=0.9695

Sigma2=acos(-C31./sind(Alpha)); %where -C31./sind(Alpha)=-0.2452

Now from the above calculation MATLAB gives

Sigma1=75.8 % Second solution manually calculated by me is 104.2

Sigma2=104.2 % Second solution manually calculated by me is -104.2

how would I automate a check for MATLAB to determine that 104.2 is the correct solution from Sigma1 and Sigma2 and then give Sigma=104.2 as the final solution. I would need this automated im gueesing using for,if,ifelse functions to do this but I'm not sure how.

Any help would be great! Thanks!

[KSSV]

Check your units of angles...

Answer 1

[Alex Mcaulley]

asind(x) %gives values between [-90,90]
acosd(x) %gives values between [0,180]

Then if you want all the values between [0,360] you can use trigonometric identities: sin(x)=sin(pi-x), and then

angle = asind(x);
angle2 = 180-a

And for the cosine cos(x)=cos(-x):

angle = acosd(x);
angle2 = 360-a

And then you can chose the one that you want.