program ex17;
var i, triangle_number : integer;
begin
triangle_number:= 0;
write ('Triangle numbers: ');
for i := 1 to 10 do
begin
triangle_number := triangle_number + i;
writeln (triangle_number);
end;
readln;
end.
We initialise a variable to 0 and then add the loop counter to it during each pass through the loop. The value of the number changes thus: 0, 0+1, 1+2, 3+3, 6+4, 10+5, etc.