The combined gas law is a mathematical relationship that describes the behavior of gases under varying conditions of pressure, volume, and temperature. It combines Boyle's law, Charles's law, and Gay-Lussac's law into a single equation. The combined gas law formula is:

```

PV = nRT

```

Where:

- P is the pressure of the gas in atmospheres (atm)

- V is the volume of the gas in liters (L)

- n is the number of moles of gas

- R is the ideal gas constant (0.08206 L*atm/mol*K)

- T is the temperature of the gas in Kelvin (K)

The combined gas law can be used to predict the behavior of a gas when any two of the three variables (P, V, or T) are known. For example, if the pressure and temperature of a gas are held constant, the volume of the gas will be inversely proportional to the pressure. Similarly, if the volume and temperature of a gas are held constant, the pressure of the gas will be directly proportional to the temperature.

The combined gas law is a useful tool for understanding the behavior of gases and can be used to make predictions about how a gas will behave under different conditions.