--> Lines like this one (beginning -->) are added comments.
--> Lines starting "$" are commands
--> Try inputs that explore all cases.

$ quadratic

--> a == 0 and b == 0

Enter a, b, and c; ctrl-D to end
0 0 1
No roots

--> a == 0 and b != 0

0 2 5
One root:  2.5

--> b*b - 4*a*c < 0

1 0 1
No real roots

--> b*b - 4*a*c == 0

Enter a, b, and c; ctrl-D to end
1 2 1
One root:  -1

--> b*b - 4*a*c > 0 (a couple of cases)

Enter a, b, and c; ctrl-D to end
1 0 -2
Two roots:  1.41421, -1.41421

Enter a, b, and c; ctrl-D to end
1 3 2
Two roots:  -1, -2

--> end with ctrl-D

Enter a, b, and c; ctrl-D to end