
theorem Th727:
Sum digits(3980,10) = 20
proof
  thus Sum digits(3980,10) = Sum <%0,8,9%>+Sum <%3%> by Th726,AFINSQ_2:55
  .= Sum <%0,8%> + Sum <%9%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 8) + Sum <%9%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 8 + 9) + Sum <%3%> by AFINSQ_2:53
  .= 17 + 3 by AFINSQ_2:53
  .= 20;
end;
