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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