考你:Can you write the formular of circle?

If we divide the circle into many equal tiny sectors,
their shape will be approximately equal to the shape of many equal tiny isosceles triangles. i.e. :


　. /\　　　　　　　　　　　　. /\
　 /. \　　approximate equal to　/. \
　/　\　　　　　　　　　　　. /＿\
　 ︶
　sector　　　　　　　　isosceles triangles
If we put the triangles like this:
　 ＿　　　　　　＿
　\　/\　　　　　\　/\
　 \. /. \　...........　\. /. \
　. \/＿\　　　　　\/＿\

a parallelogram will form,
then the base of it is circumference/2=2 π r/2=π r, where r is the radius, π is "pi",
and its height approximately equal to radius=r
　 ＿　　　　　　＿
　\　/\　　　　　\　/\
　 \. /. \　...........　\. /. \　r
　. \/＿\　　　　　\/＿\
　　<---------- π r -------->

The area of the parallelogram =base x height=π r x r = π r²=The area of circle

So,The area of circle= π r²

3.1416 X radius X radius