168=3×8×7
a=7+7^2+...+7^2016=
avem 2016 termeni putem sa-i grupam cate 6 (2016:6=336)
=7+7^2+7^3+7^4+7^5+7^6+....+
7^2010+7^2011+7^2012+7^2013+7^2014+7^2015)=
=(7+49+343+2401+16807+117649)+
+7^2009(7+49+343+2401+16807+117649)=
=137256×(1+....+7^2009)=
=168×817×(1+....+7^2009) deci divizibil cu 168
