This page covers the basics of cylindrical coordinates and reference equations that use cylindrical coordinates for multivariable calculus.
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When you studied polar coordinates, you learned that you could take an area in the plane in rectangular coordinates and use polar coordinates to describe the same area. To do this, you used the equations \(x=r\cos(\theta)\) and \(y=r\sin(\theta)\). These equations convert the equations for an area in the plane from rectangular coordinates \((x,y)\) to polar coordinates \((r,\theta)\).
\(x=r\cos(\theta)\) 
\(y=r\sin(\theta)\) 
\(z=z\) 
\(r^2=x^2+y^2\) 
Table 1 

However, now we have three dimensions. In rectangular coordinates, we have \((x,y,z)\). One of the nice things about cylindrical coordinates is that we use the same equations on x and y that we used for polar coordinates to get r and \(\theta\) and to go to cylindrical coordinates z does not change. Another way of looking at it is that we take polar coordinates \((r,\theta)\) and slap on the rectangular coordinate z to the end to get \((r,\theta,z)\) and call this cylindrical coordinates. So the cylindrical coordinates conversion equations are given in Table 1 and Figure 1 shows this relationship.
Figure 1 

[source: Paul's Online Notes] 
Figure 2 

[source: Wikiversity  Cylindrical Coordinates] 
Comment About Notation  Figure 2 shows another way to describe cylindrical coordinates. Some books, instructors, videos and sites use \((\rho,\phi,z)\) to describe the same point as \((r,\theta,z)\). We choose to use \((r,\theta,z)\) for cylindrical coordinates since the cylindrical coordinate system is so closely related to twodimensional polar coordinates usually described at \((r,\theta)\). As always, check with your instructor to see what they expect you to use.
Also in Figure 2, they are calling A the polar axis (positive xaxis) and L is the positive zaxis.
Del Operator and Cylindrical Coordinates
video by Michel vanBiezen 

Really UNDERSTAND Calculus
external links you may find helpful 

Wikipedia  Connection between polar coordinates and spherical and cylindrical coordinates 
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