hello
in dual slope I am stuck in understanding one thing
why the capacitor start discharging when the switch switches from the voltage of X to the ref voltages !
what is the reason that makes the capacitor discharge ?

 

Typically the capacitor charges with a current proportional to the input voltage for a fixed time and then is discharged by a constant-current source controlled by a reference voltage for the same fixed time.
From that you can calculate the input voltage with reasonably good accuracy, and the tolerance of the capacitor value has no effect on that accuracy.