The circumference of a circle is found by the relation `C=pid` or `C=2pir` where d is the diameter of the circle and r is the radius of the circle.

So the circumference of a circle is directly proportional to the diameter, with the constant of proportionality `pi` .

From anicent greeks the came to know that the raztio between a cirucmference and the radius of , is costant. The problem was greek didn't known no integer numbers yet. So another problem to find the ratio betrween a diagonal value of a a square and its side was too much hard.Times went by, they come to know not only the ratio between half arc of cirumference C was costant, but its value:

`C/r=3,1415926535897932384626433832795.......`

the points mean the number keeps on have cifres not equal zero.

tsi number is called `pi` , and wil be found in many application of math, physich and so on.