#mimetex( \Bigsum_{n=0}^\infty ar^{n-1}=\frac{a}{1-r} )
ただし|r| < 1
例)
#mimetex( \frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+...=1 )
http://www.nikonet.or.jp/spring/mugen/Img001.gif
[[数学のいずみ:http://www.nikonet.or.jp/spring/mugen/mugen.htm]]