Geometry · real student question

Convert the cylindrical coordinates (r, theta, z) = (2, 30 degrees, 5) to Cartesian coordinates.

Question

Convert the cylindrical coordinates

(r,θ,z)=(2,30,5)(r,\theta,z)=\left(2,\,30^\circ,\,5\right)

to Cartesian coordinates (x,y,z)(x,y,z).

Step-by-step solution

  1. Recall the conversion equations. Cylindrical coordinates are polar coordinates in the xyxy-plane with an unchanged height:

    x=rcosθ,y=rsinθ,z=zx=r\cos\theta,\qquad y=r\sin\theta,\qquad z=z

    The third coordinate needs no work at all — the whole conversion happens in the plane.

  2. Substitute the exact trigonometric values for 3030^\circ.

    cos30=32,sin30=12\cos 30^\circ=\frac{\sqrt3}{2},\qquad \sin 30^\circ=\frac12

    Using exact surds rather than decimals keeps the answer exact.

  3. Compute xx and yy.

    x=232=31.7321,y=212=1x=2\cdot\frac{\sqrt3}{2}=\sqrt3\approx 1.7321,\qquad y=2\cdot\frac12=1

  4. Carry the height across and state the point.

    (x,y,z)=(3,1,5)(1.7321,1,5)(x,y,z)=\left(\sqrt3,\,1,\,5\right)\approx(1.7321,\,1,\,5)

  5. Check by converting back. The radial distance is x2+y2=3+1=2=r  \sqrt{x^2+y^2}=\sqrt{3+1}=2=r\;\checkmark, and the angle is arctan ⁣(13)=30  \arctan\!\left(\tfrac{1}{\sqrt3}\right)=30^\circ\;\checkmark — in the first quadrant, as the positive xx and yy require. Both round-trip checks confirm the conversion.

Answer

(x,y,z)=(3,1,5)(1.7321,1,5)(x,y,z)=\left(\sqrt3,\,1,\,5\right)\approx(1.7321,\,1,\,5)

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