
theorem Th527:
Sum digits(1980,10) = 18
proof
  thus Sum digits(1980,10) = Sum <%0,8,9%>+Sum <%1%> by Th526,AFINSQ_2:55
  .= Sum <%0,8%> + Sum <%9%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 8) + Sum <%9%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 8 + 9) + Sum <%1%> by AFINSQ_2:53
  .= 17 + 1 by AFINSQ_2:53
  .= 18;
end;
