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# Line integral help needed watch

1. I am not sure how to find the line segment dl to sub into the formula. What I tried was writing dl=dx î+dyĵ +dzk̂ so subbing in x=rcosθ and y=rsinθ and z=z (cylindrical coordinates) gives dl=(-sinθ î+cosθ ĵ + k̂)dθ
but this does not give the correct answer so I assume this dl is incorrect?

EDIT: correct question uploaded. THe one I'm looking at is 3c
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2. Circle of unit radius in the (x-z) plane means where t is between 0 and 2pi.
3. (Original post by Math12345)
Circle of unit radius in the (x-z) plane means where t is between 0 and 2pi.
Thanks! Got it now. But does that mean that the polar co ordinate definitions x=rcostheta and y=rsintheta isn't "strict" so you can e.g. let z- rcostheta or rsintheta and y=0 depending on the question?
4. (Original post by bobbricks)
Thanks! Got it now. But does that mean that the polar co ordinate definitions x=rcostheta and y=rsintheta isn't "strict" so you can e.g. let z- rcostheta or rsintheta and y=0 depending on the question?

Yep, it depends which plane the circle is in.

For example if the plane (half-plane) is then the semi-circle of radius 2 lying in that half-plane would be parameterised by where t is between 0 and .

The above parameterisation works because the equation of a circle of radius 2 in 3d is which is in the plane. Now sub in the components to see this is satisfied.

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