# Circumference and Area of circle

• Oct 18th 2011, 11:26 AM
Zyzz
Circumference and Area of circle
Why is it that the circumference of the circle is given by 2rPi which is the derivative of r^2Pi by r, which gives the area of it. Same if we transfer it to cartesian coordinates, y= Sqrt(r^2-x^2) the arc length is given by r(arcsin(x/r)) which is the derivative of r(arcsin(x/r) + r/2(Sqrt(r^2-x^2))) which on the other hand gives the area of the quarter circle with radius r. The same applies to the sphere. Can this be extended to the n-dimensional circle? (sphere: V=4/3Pir^3 SA= 4Pir^2)

its just something interesting i found today and was wondering abt the explanation behind this.
• Oct 18th 2011, 11:36 AM
Zyzz
Re: Circumference and Area of circle
anyone?
• Oct 18th 2011, 11:56 AM
emakarov
Re: Circumference and Area of circle
Googling for "circumference is derivative of area" gives a lot of results.
• Oct 18th 2011, 12:03 PM
Zyzz
Re: Circumference and Area of circle
right. i couldve done that too, but i was looking for a more formal reasoning from someone who knows what hes talking about not yahoo answers with 13 year old kids so i came here.