Question 313527
The distance from A to B would be the diameter.
Use the distance formula,
{{{ D^2=(x2-x1)^2+(y2-y1)^2}}}
{{{D^2=(a-a)^2+(2b-0)^2}}}
{{{D=2b}}}
Then the area of a circle in terms of the diameter, D, is,
{{{A=(pi/4)*D^2}}}
{{{A=(pi/4)*(2b)^2}}}
{{{A=(pi/4)*4b^2}}}
{{{A=pi*b^2}}}