求圓形面積解

It is because you calculate the area using height X length.
A circle can become these two thing
︵︵︵︵
.\/ \/ \/ \/
./\ /\ /\ /\
︶︶︶︶
these two thing can become -
︵︵︵︵ ＜---- piXr = length
.\//\\//\\//\\//\＜---- r = height
..︶︶︶︶＜---- piXr = length
this time- the length= piXr
the height= r

and heightX length= pi X r X r

can i explain it by using integration -.-!?

2009-06-01 19:01:53 補充：
Here is the prove using integration(of course, definte integrals):
http://i146.photobucket.com/albums/r254/kwanhimshek/circle_area_in.jpg

Yes...using integration is an appropriate method to prove the circle area=pi*r^2

i don't want to answer because i may not answer ggod,
you may see this web site:

http://cd1.edb.hkedcity.net/cd/maths/tc/ref_res/ld_c/Circlec.pdf