Circle centered at origin (0,0):
Circle centered at (h,k): use distance formula:
√((x-h)2+(y-k)2)=r
General form equation: x2+y2-2hx-2ky+(h2+k2-r2)=0
Area of a circle = πr2. Circumference = 2πr. d=2r, π=C/d ratio of circumference to diameter.
Double the radius → quadruple the area and double the circumference.
Circle is the shape that encloses the most area for a given "perimeter".
C=1 → A=1/(4π)≈0.079577 (cf square of side 1/4 has area 1/16=.0625)