
theorem Th521:
Sum digits(1920,10) = 12
proof
  thus Sum digits(1920,10) = Sum <%0,2,9%>+Sum <%1%> by Th520,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%9%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%9%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 2 + 9) + Sum <%1%> by AFINSQ_2:53
  .= 11 + 1 by AFINSQ_2:53
  .= 12;
end;
