
theorem Th1170:
Sum digits(1875,10) = 21
proof
  thus Sum digits(1875,10) = Sum <%5,7,8%>+Sum <%1%> by Th1169,AFINSQ_2:55
  .= Sum <%5,7%> + Sum <%8%> + Sum <%1%> by AFINSQ_2:55
  .= (5 + 7) + Sum <%8%> + Sum <%1%> by AFINSQ_2:54
  .= (5 + 7 + 8) + Sum <%1%> by AFINSQ_2:53
  .= 20 + 1 by AFINSQ_2:53
  .= 21;
end;
