Converting Coordinates from Rectangular to Polar

Key Questions

  • Wikimedia

    Let's look at the trig formulas SYR, CXR, TYX:

    #sin theta = y/r#
    #cos theta = x/r#
    #tan theta = y/x#

    Since we are given the Cartesian coordinates, we are given #x# and #y#. For polar coordinates, we need to figure out #r# and #theta#. #r# is easy, we just use Pythagorean:

    #r=sqrt(x^2+y^2)#

    To figure out #theta#, I like to use cosine because the range of arccosine is in quadrants I and II and adjusting #theta'# is easier. So,

    #theta'=cos^(-1)x/r#

    If #y>=0# then #theta=theta'#.
    If #y<0# then #theta=2 pi - theta'# (in radians) or #theta=360-theta'# (in degrees).

    Our final answer is #(r, theta)#.

    Let's look at a concrete example: Convert #(-3, 3sqrt3)# to polar coordinates:

    #r=sqrt((-3)^2+(3sqrt3)^2)=sqrt(36)=6#
    #theta'=cos^(-1)((-3)/6)=(2pi)/3#
    #y<0# so, #theta=2pi-(2pi)/3=(4pi)/3#

    So the polar coordinates are #(6, (4pi)/3)#.

  • I presume we're looking for a radius #r# and angle #theta# such that #a + bi = r(cos theta + i sin theta)#.

    Pythagoras theorem gives us #r = sqrt(a^2+b^2)#.

    Simple trigonometry gives us #tan theta = b/a#, so #theta = arctan (b/a)#.

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