1/3+1/6+1/18+1/36+.+1/216
=(1/3)×(1+1/2+1/6+1/12+.+1/72)
=(1/3)×(1+1-1/2+1/2-1/3+1/3-1/4+.+1/8-1/9)
=(1/3)×(2-1/9)
=17/27