These are harmless memory resident parasitic virus. They hook INT 21h and write themselves to the end of COM- and EXE-files that are executed. They cure the infected files on file opening. They contain the internal text strings:
SD93
Quadratic Equation
"Quadratic.981,986" hook INT 09h also (keyboard) and sometimes display the message "Quadratic Equation" on DEL key pressing.
"Quadratic.1283" is a polymorphic virus. It contains the internal string "Quadratic Equation II".
