What is the equation of a circle with a radius2 units?

Answer 1

Equation of any circle centre (xo, yo) and radius r is:

(x - xo)2 + (y - yo)2 = r2

You give no centre for you circle, so centred on (xo, yo) its equation is:

(x - xo)2 + (y - yo)2 = 22

⇒ (x - xo)2 + (y - yo)2 = 4

If your circle is centred on the origin (0,0), this becomes:

x2 + y2 = 4

If it's centred on, say, (1, 2) it becomes:

(x - 1)2 + (y - 2)2 = 4

⇒ x2 + y2 - 2x + 4y + 1 = 0