
theorem Th665:
Sum digits(3360,10) = 12
proof
  thus Sum digits(3360,10) = Sum <%0,6,3%>+Sum <%3%> by Th664,AFINSQ_2:55
  .= Sum <%0,6%> + Sum <%3%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 6) + Sum <%3%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 6 + 3) + Sum <%3%> by AFINSQ_2:53
  .= 9 + 3 by AFINSQ_2:53
  .= 12;
end;
