很简单,适当放缩即可：
c1+c2+...+cn=1/2+1/8+1/24+1/64+1/160+1/384+...<1/2+1/8+1/24+1/64+(1/160+1/320+1/640+...)=1/2+1/8+1/24+1/64+(1/160)(1+1/2+1/2^2+...)
而1+1/2+1/2^2+...=2-1/2^(n+1)<2
所以1/2+1/8+1/24+1/64+(1/160)(1+1/2+1/2^2+...)<1/2+1/8+1/24+1/64+1/80=667/960≈0.695<7/10
证毕.