利用等比数列的求和公式：
1/2+1/2^2+1/2^3+…+1/2^10
=1/2(1-(1/2)^10)/(1-1/2)
=1-(1/2)^10
=1023/1024

1/2+1/2^2+1/2^3+…+1/2^n
=1/2(1-(1/2)^n)/(1-1/2)
=1-(1/2)^n