Symbolic acos( cos(theta) ) does not return theta.
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While cos(acos(x)) correctly returns x, acos(cos(theta)) does not return theta.
syms theta real
assume( theta >-1 & theta < 1)
a = simplify( acos(cos(theta)) )
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Walter Roberson
2023-3-31
syms theta real
assume( theta >-1 & theta < 1)
a = simplify( acos(cos(theta)) )
fplot(a, [-1 1])
If the identity holds then you would expect a straight line, not two lines.
But cos(-theta) = cos(theta) so cos(-1) = cos(1) and so acos(cos(-1)) = acos(cos(1)) rather than being able to distinguish -1 and 1
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John D'Errico
2023-3-31
编辑:John D'Errico
2023-3-31
Is it true, that acos(cos(theta)) ALWAYS returns theta? TRY AN EXAMPLE.
acos(cos(10))
So it is not true. It works the other way of course. at least it is mathematically true.
But the range of acos is [0,pi) for real arguments, so it cannot return a number outside that interval.
fplot(@acos,[-1,1])
And so then acos(cos(theta)) is possibly best written as
abs(pi - mod(theta - pi,2*pi))
At least, that is the best I can find for an approximation.
fplot(@(theta) acos(cos(theta)),[-20,20],'r--')
hold on
fplot(@(theta) abs(pi - mod(theta - pi,2*pi)),[-20,20],'b:')
You can see the curves overlay on top of each other.
We can try syms, but simplify won't find that solution, even if I push it pretty hard, harder than what I did below:
syms T
simplify(acos(cos(T)),'steps',10,'all',true)
Anyway, the point is, MATLAB will not return the result you expect there, because it is not true.
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