
theorem Th473:
Sum digits(1440,10) = 9
proof
  thus Sum digits(1440,10) = Sum <%0,4,4%>+Sum <%1%> by Th472,AFINSQ_2:55
  .= Sum <%0,4%> + Sum <%4%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 4) + Sum <%4%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 4 + 4) + Sum <%1%> by AFINSQ_2:53
  .= 8 + 1 by AFINSQ_2:53
  .= 9;
end;
