Change from rectangular to cylindrical coordinates: (1,-2,1)-- (r,theta,z)

Question

Change from rectangular to cylindrical coordinates: (1,-2,1)--> (r,theta,z)

anonymous · Answer

$$r=\sqrt{5}$$

anonymous · Answer

z=1

anonymous · Answer

but what is theta. I know you use ArcTan[-2/1]=Theta but I need the exact value

amistre64 · Answer

isnt theta an xy plane rotation while the last one is an zy plane rotation?  perse

amistre64 · Answer

theta = tan y/x

anonymous · Answer

yeah but i get a decimal value I need it in Pi

amistre64 · Answer

arctan(-2) IS an "exact" value

oh, then divide out the pi

anonymous · Answer

yeah i know that, I just wasn't sure how to word what I am asking for.  It is an internet based homework and when I enter ArcTan[-2] it is wrong and i don't think it wants the estimated value either cause it would have asked.

amistre64 · Answer

1pi = 3.14159...

1 = 3.14159... / pi

amistre64 · Answer

hmmm

anonymous · Answer

yeah its odd. Is there another way to find the angle?

anonymous · Answer

attachment

amistre64 · Answer

http://www.wolframalpha.com/input/?i=%28arctan%28-2%29%29*180%2Fpi another way? no. at least nothing that will give you a rational value

anonymous · Answer

oh and the 14pi/4 was a crazy guess off terrible logic. ignore that.

amistre64 · Answer

arctan is gonna give you a value less then 0 so we would have to add 2pi to it to get a value between 0 and 2pi, for starters

anonymous · Answer

oh hmm okay i could try ArcTan[-2]+2pi but I will be out of trys after this one.

amistre64 · Answer

|dw:1333053831067:dw|

anonymous · Answer

Nice, it worked. Thanks. I should have paid attention to the question a little more.

amistre64 · Answer

:)  good job