Convert Equation from Rectangular to Polar Form


Problems were equations in rectangular form are converted to polar form, using the relationship between polar and rectangular coordinates, are presented along with detailed solutions.

Problem 1: Convert the equation

2x 2 + 2y 2 - x + y = 0


to polar form.

Solution to Problem 1:

  • Let us rewrite the equations as follows:

    2 ( x 2 + y 2 ) - x + y = 0

  • We now use the formulas giving the relationship between polar and rectangular coordinates: R 2 = x 2 + y 2, y = R sin t and x = R cos t:

    2 ( R 2 ) - R cos t + R sin t = 0

  • Factor out R

    R ( 2 R - cos t + sin t ) = 0

  • The above equation gives:

    R = 0

    or

    2 R - cos t + sin t = 0

  • The equation R = 0 is the pole. But the pole is included in the graph of the second equation 2 R - cos t + sin t = 0 (check that for t = Pi / 4 , R = 0). We therefore can keep only the second equation.

    2 R - cos t + sin t = 0

    or

    R = (1 / 2)(cos t - sin t)


Problem 2: Convert the equation

x + y = 0


to polar form.

Solution to Problem 2:

  • Use y = R sin t and x = R cos t into the given equation:

    x + y = 0

    R cos t + R sin t = 0

  • Factor out R

    R ( cos t + sin t ) = 0

  • The above equation gives:

    R = 0

    or

    cos t + sin t = 0

  • The equation R = 0 is the pole. But the pole is included in the graph of the second equation cos t + sin t = 0 since this equation is independent of R. We therefore keep only the second equation.

    cos t + sin t = 0

  • The above equation may be written as.

    tan t = - 1

  • Solve for t to obtain

    t = 3 pi / 4

  • All points of the form (R , 3 pi / 4) are on the graph of the above equation. It is the equation of a line in polar form.

More references on polar coordinates and trigonometry topics.


Polar Coordinates.

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