Question 1005218
.
see

<A HREF=http://www.mathopenref.com/circleareaderive.html>http://www.mathopenref.com/circleareaderive.html</A>

or

<A HREF=https://www.math.hmc.edu/funfacts/ffiles/20009.2-3.shtml>https://www.math.hmc.edu/funfacts/ffiles/20009.2-3.shtml</A>