The question cannot be answered because it does not state clearly what the 81 refer to and what you are trying to find.
To find out the area of a circle.
Area = (pi) (R)2
answer
It is: diameter/2 = 6 cm
area of sector = (angle at centre*area of circle)/360
Pi is used to find area and circumference of a circle.
It is used to find the area of a circle as pi*radius2
Area of the circle in square inches = pi*82
If the radius is 42 meters, then you already know the radius.
Area in square units = pi*radius2
You are thinking of pi.A = (pi)r^2
Pi is used in many ways today. The simplest way it to find the circumference (the perimeter) of a circle or the area (amount of space inside) of a circle.