
theorem Th725:
Sum digits(3960,10) = 18
proof
  thus Sum digits(3960,10) = Sum <%0,6,9%>+Sum <%3%> by Th724,AFINSQ_2:55
  .= Sum <%0,6%> + Sum <%9%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 6) + Sum <%9%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 6 + 9) + Sum <%3%> by AFINSQ_2:53
  .= 15 + 3 by AFINSQ_2:53
  .= 18;
end;
