why do we have to use phi= arccos(z/rho) instead of arcsin(r/rho)? my professor told that we need to use arccos for some reason and to think about it . I don't see any differences.
Yes, you are right actually there is no difference between two. You will get same value of phi from both the cases.
But It is convenience to write in this way, or rather easy or easy to remember. Sometimes we get confused between and . As in spherical coordinate system we user r as radius which
and in cylindrical coordinate system
and
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