用等比数列求和公式减1/2+1/41/2+1/4+1/8+1/16+1/32+1/64+1/128+1/256+.....+1/2的N 次幂=[1/2(1-1/2^n)]/1-1/21/8+1/16+1/32+1/64+1/128+1/256+.....+1/2的N 次幂的最后得数是用{[1/2(1-1/2^n)]/1-1/2}-1/2-1/4
