Dear Student,
Let two capacitors C1 and C2 are connected in series combination and the applied voltage is V. In a series combination the magnitude of charge (q) on all plates is same. We know that in series combination of capacitors the total capacitance is given by formula-
1/C = 1/C1 + 1/C2
q/C = q/C1 + q/C2
V = V1 + V2
As the potential is distributed in the inverse ratio of the capacity (V α 1/C or V = q/C)
Thus in series combination of capacitors the sum of the voltage across all the capacitors is equal to the sum of the voltage of the circuit.
Regards.