Implementation of the algorithm solving a quadratic equation in Pascal

<<algorithm

program Quadratic_equation;

procedure linear(a: double; b: double);
var x: double;
begin
  if a=0 then
    if b=0 then
      writeln('identity equation')
    else
      writeln('contrary equation')
  else begin
    x:=-b/a;
    writeln('Linear equation, x = ',x);
  end;
end;

var a,b,c,delta,x,x1,x2: double;
begin
  write('a = ');
  readln(a);
  write('b = ');
  readln(b);
  write('c = ');
  readln(c);
  if a=0 then 
    linear(b,c)
  else begin
    delta:=b*b-4*a*c;
    if delta<0 then
      writeln('lack of solutions')
    else
    if delta=0 then begin
      x:=-b/(2*a);
      writeln('one solution x = ',x);
    end else begin
      x1:=(-b+sqrt(delta))/(2*a);
      x2:=(-b-sqrt(delta))/(2*a);
      writeln('x1 = ',x1);
      writeln('x2 = ',x2);
    end;
  end;
end.

Program line	Description	Block algorithm
1	Every program in the pascal must start with the keyword program preceding the name of the program, it adds nothing to the implementation of the algorithm.	-
3 - 15	We declare and define a linear procedure that implements the algorithm of solving a linear equation.	4
17	We declare all variables used in the main program. In a pascal, we must do this before the keyword begin, which starts the operating part of the code.	-
18	The beginning of the program code.	1
19 - 24	On the screen, we print the prompts for the user and load the values of the coefficients a, b and c entered into the corresponding, previously declared, variables.	2
25	We check if the coefficient a is equal to 0. If this is the case with the linear equation.	3
26	In this line we will only find ourselves if a = 0, ie when the equation is a linear equation. We invoke procedure linear(b, c) that realizes the algorithm of solving a linear equation. It is worth noting that the coefficient at the first power of the variable x is marked with the symbol b, and the free expression c (traditionally in the linear equation, a and b are used respectively).	4
27	else, i.e. otherwise. At this point, the solution of the equation begins, when a ≠ 0, in other words when we actually deal with a quadratic equation.	-
28	We calculate the value of the delta variable.	5
29	We check if delta < 0.	6
30	When delta < 0, we print an appropriate message about the lack of solutions.	7
31	Here the program block begins when the delta is not less than zero, ie when the equation has solutions (one or two).	-
32	We check if delta = 0.	8
33	We calculate and store in the x variable the solution of the equation (single, double - delta=0).	9
34	We display the calculated result.	10
35	We start the program block executed in the situation when the condition delta = 0 (and earlier delta <0 and a ≠ 0) was not met.	-
36 - 37	We calculate solutions and their values we assign to variables x1 and x2.	11
38 - 39	We display the calculated result - two solutions of the quadratic equation.	12
40 - 41	We close the blocks of compound instructions.	-
42	The end of the program implementation.	13