Which of the following parametric equations are equivalent to the polar equation r=3 cos theta

To solve the problem you must know that to change from Cartesian to polar coordinates you must write:
 x = rcos (θ)
 Where "r" is the radius
 Likewise y = rsin (θ)
 Therefore, in the expression r = 3cos (θ) one can write r as:
 r = x / cos (θ)
 So:
 x / cos (θ) = 3cos (θ)
 x = 3cos ^ 2 (θ)
 The same for y ...
 y = rsin (θ)
 y = 3cos (θ) sin (θ).

  Finally the correct answer is option 2.
 x = 3cos ^ 2 (θ) y = 3cos (θ) sin (θ).