reconsider e = x as Point of (Euclid n) by TOPREAL3:13;
A1: ( r is Real & Ball (e,r) is bounded ) by XREAL_0:def 1;
A2: n in NAT by ORDINAL1:def 13;
then Ball (e,r) = Ball (x,r) by Th13;
hence Ball (x,r) is open by A1, A2, GOBOARD6:6; :: thesis: verum