Radix (radic-is), a root; radical, eradicate.
Rapio (rapt-um), I seize; rapture, rapine, surreptitious.
Ratio, reason; rational, ration, reason.
Rego (rect-um), I rule; regiment, regal, regulate, rector, rectify.
Res (re-i), a thing; real, reality, republican.
Rex (reg-is), a king; regal, interregnum, royal.
Rideo (rīs-um), I laugh; deride, derision.
Rivus, a brook; river, rival.
Rodo (ros-um), I gnaw; erosion, corrode.
Rogo, I ask; derogatory, interrogation, arrogate.