Voltage Divider
Output voltage will be a ratio of the two resistances and the input voltage:
\[Vout = \frac{R_2}{R_1+R_2}*Vin\]Notes: assuming that Vin output impedance and Vout input impedance are respectivelly low impedance and high-impedance compared to R1+R2
Complex impedance
Considering Laplace Transforms and complex impedance we can rewrite this as :
\[Vout = \frac{Z_2}{Z_1+Z_2}*Vin\]When Z1 is a capacitor and Z2 is a resistor we have a low pass filter
When Z1 is a resistor and Z2 is a capacitor we have a high pass filter