Ive been working through calculus this year and will be into next year, and as nearly every time I open my calculus book I have found something new and mysterious. This time it's something called parametric representation. It isn't clearly explained what this means or how you go about converting between implicit/explicit and parametric forms. An example of what my book is giving me. X^2 +Y^2 = 9 is the equation of a circle with center at the origin and radius equal to 3. If [theta] is the angle that the radius to the point (x,y) on the circle makes with the x axis, find the parametric representation of the circle in terms of [theta]. What rules govern this subject? Why do I need it? How and when do I use parametric representation? Many thanks in advance.