Area ratio of nozzle
The area ratio for a nozzle with isentropic flow can be expressed in terms of Mach numbers for any points x and y within the nozzle.
Considering a rocket nozzle, we can set the mass flow rate by setting the area of the throat. And we can set the exit Mach number by setting the area ratio of the exit to the throat. Using the isentropic relations, we can determine the pressure and temperature at the exit of the nozzle.