This is basic question. Since childhood I am mugging the mathematical formulae areas of square, rectangle and circle etc.
Now,it is possible for me to understand formula of area of square I.e. square of length, area of rectangle as product of length and breadth.
But I still don't understand formula for area of circle I.e. $A=\pi r^2$
I want to know that
How do mathematicians invented the "$\pi$" term?
How do they introduced "$\pi$" term in the case of the circle? (circumference and area of the circle)