解法2：
program ling(input,output);
var a,b,c,d,x,y,z:real;
begin
writeln('This is a program to calculate the quadrie equation with one unknow.');
writeln('From:ax*x+bx+c=0');
writeln;
write('Please input data far a,b,c(for example:1 2 3):');
read(a,b,c);
if (a=0)and(b=0)and(c=0) then
writeln('the calculate result is:x can be all real numbers');
if (a=0)and(b=0)and(c<>0) then
writeln('the calculate result is:this equation has no answer');
if (a=0)and(b<>0)and(c<>0) then
begin
x:=-c/b;
y:=-c/b;
writeln('x1=x2=',x:8:1);
end
else
if (a<>0) then
begin
z:=sqr(b)-4*a*c;
 if (z<0) then
 writeln('thecalculate result is:this equation has no answer');
if (z>=0) then
begin
x:=(-b+sqrt(z))/(2*a);
y:=(-b-sqrt(z))/(2*a);
writeln('x1=',x:8:2);
writeln('x2=',y:8:2);
end
end
end.

结果验证：

运行时，输入：

0 0 0 提示：x is real

0 0 1 提示：no result

0 1 2 提示：x1=x2=-2.00

1 2 9 提示：no result

1 9 3 提示：x1=-0.35
                      x2=-8.65

1 2 1 提示：x1=-1.00
                      x2=-1.00

4 4 1 提示：x1=-0.5
                      x2=-0.5

4 4 -1 提示：x1=0.21
                       x2=-1.21

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举一反三，融会贯通