The capacitive reactance of a capacitor connected to 50 V d.c. is

An air capacitor of plate area ' $A$ ' and separation between the plates is ' $d$ ' has a capacity ' $C$ '. Two dielectric slabs are inserted between its plates in two different manners as shown in figure. The capacitance of a capacitor is

When three capacitors of capacitances ' C ', ' 2 C '? and ' 3 C ' are connected in series. The potential difference applied across the combination is ' $V$ '. The energy stored in the circuit is

In between the plates of parallel plate capacitor of plate separation 'd' a dielectric plate of thickness ' $t$ ' is inserted. The capacitance becomes one-third of the original capacity without dielectric. The dielectric constant of the plate is