Question

What is pi? Why do we use in circle? Why do we not use any other formula to find area or perimeter of circle?

Answer 1

The number π is a mathematical constant. Originally defined as the ratio of a circle's circumference to its diameter, it now has various equivalent definitions and appears in many formulas in all areas of mathematics and physics. It is approximately equal to 3.14159. It has been represented by the Greek letter "π" since the mid-18th century, though it is also sometimes spelled out as "pi".

Being an irrational number, π cannot be expressed exactly as a common fraction,Still fractions such as 22/7 and other rational numbers are commonly used to approximate π.
Since it is defined mathematically as the ratio of circumference of circle by diameter,it is used in the every equation of circle.also circle has no sides so that no other equation can be used.