Using formulas to explain why contraction of blood vessels leads to increased blood pressure
There is an important missing assumption in your statement, I've added it in italics:


  Contraction of blood vessels leads to increased blood pressure if total flow stays constant.


All you need is your first equation:

Flow = Pressure/Resistance

Decreasing the radius increases the resistance. If flow is constant, then pressure has to increase to get the same flow through the smaller vessel.