a(n+1)=1/2*an
a(n+1)/an=1/2
所以an是以2为首项,公比为1/2的等比数列
an=a1q^(n-1)
=2*(1/2)^(n-1)
=(1/2)^(n-2)
s5=a1(1-q^5)/(1-q)
=2*[1-(1/2)^5]/(1-1/2）
=(2*31/32)/(1/2)
=31/16*2
=31/8