1/2+1/4=3/41/2+1/4+1/8=7/81/2+1/4+1/8+1/16=15/16......1/2+1/4+1/8+1/16+……+1/1024+1/2048=2047/2048
1/2 + 1/2^2 + ... + 1/2^11= 1-1/2^11
