# What is the proper form of the combined gas law?

$\frac{P V}{T} = k \text{ (a constant)}$
$\frac{P V}{T} = k \text{ (a constant)}$
$\frac{{P}_{1} {V}_{1}}{T} _ 1 = \frac{{P}_{2} {V}_{2}}{T} _ 2$