SNS3Guppy; I stand corrected and have edited my previous post.
However, on the continuing saga of what provides the decelleration force from a thrust reverser...
Please explain how you have managed to remove "thrust" from your formula. Indeed, if you were to turn-off the fuel supply, you'd produce drag from a windmilling engine; however, this is not the case. If you were to shut-down an engine in-flight there isn't a huge ammount of drag to overcome. Now do the same with a reverser deployed; yes, there would be an increase in drag, but not as much as there would be with the engine running.
If, during flight, you experience an inadvertant thrust reverser deployment (accomanied by loss of airspeed, buffeting/vibration), most (if not all) Emergency Checklists instruct you to shut-down the engine. Why do you think this is? In this example we've removed the "thrust" and regained control of the aeroplane. Your theory implies that the removal of the thrust would further increase the drag!
With regard to thrust reverser design... indeed, not all thrust reverser systems divert the thrust in a forward direction (usually perpendicular to the free stream air); however, the net effect is the same. As Mustafagander has said, this produces a "barn door" type of effect to the free-stream airflow and, as trimotor has correctly stated, is proportional to TAS.
Happy New Year
TCF