they can be derived by using calculus(integration to be precise)
[tex] \pi [/tex][tex]= \frac{circumference }{diameter} [/tex](its how pi is defined)
so that gives circumference
to get area , you use integration [tex]of an elemental ring of radius r and thickness dr and integrate it[/tex][tex] A= \int\limits^0_r {2 \pi r} \, dr [/tex]
that gives the area.
if you don't know calculus yet ,dont worry, just remember the formulae.
