Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It only takes a minute to sign up.
Sign up to join this community
Find w, the angle ∠rox. That is, the angle from the red vector r to the positive x axis, in terms of u and v.
Also find the derivative of w when neither u nor v are held constant but instead are held to remain proportional. That is, doubling u will double v.
This is seeking to prove or disprove that if u and v are held proportional to each other that w changes with them, proportionally.
We can directly project vector $ \vec or $ on positive $x-$ axis as
$$ \cos \angle rox =(\cos v \cos u) $$
It is in effect the same when we take dot product of vectors
$$ \vec {ox}= (\cos v \cos u,0,0);\; \vec {or} = (\cos v \cos u,\cos v \sin u,\sin v) $$
$$\cos w_1=\cos ∠rox =\dfrac{(\cos v \cos u)^2+0+0}{(\cos v \cos u)\cdot (1) }=\cos v \cos u $$
$$\cos w_2=\cos 2v \cos 2u.$$
If $ v=um,$ then $ (\cos m v \cos u)$ is not proportional to $ (\cos v \cos u) $.
For the double angles
If e.g., $ u=v=\pi/3, \cos w_1=\dfrac14; \cos w_2= \dfrac14 $ which does not even change.
There can be no linearity or proportionality with circular trigonometric ratios products. Can be also confirmed by graphing the two cosines.
I would say, $\,\cos ∠rox = \frac{r\cos v \cos u}{r}=\cos v \cos u$
