asin and acos quadrant check function help

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Adam Francki
Adam Francki 2019-3-19
回答： Alex Mcaulley 2019-3-19
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!

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回答（1 个）

Alex Mcaulley
Alex Mcaulley 2019-3-19
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.

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