1-1/2+1/4-1/8+1/16-1/32+1/64-1/128+1/128+1/256-1/512

不妨将每两项看做一个数,即a1=1/2,a2=1/8=(1/2)^3,a3=1/32=(1/2)^5……an就是一个首项为1/2,公比为1/4的等比数列.an=1/2×1/4^(n-1);Sn=1/2(1-1/4^n)/(1-1/4) =2/3*(1-1/4^n)把n换成5就是答案.1-1/2+1/4-1/8+1/16-1/...等比没学过，预初，别的方法有吗1-1/2+1/4-1/8+1/16-1/32+1/64-1/128+1/256-1/512=1/2+1/8+1/32+1/128+1/512=(256+64+16+4+1)/512=341/512=0.666015625