State Carnot's theorem.

(N/A) Carnot's theorem states two main points regarding the efficiency of heat engines:
$1$. All reversible heat engines operating between the same two temperatures ($T_1$ and $T_2$) have the same efficiency,regardless of the working substance used.
$2$. No irreversible heat engine operating between two given temperatures can have an efficiency greater than that of a reversible heat engine operating between the same two temperatures.
Mathematically,the efficiency $\eta$ of a Carnot engine is given by $\eta = 1 - \frac{T_2}{T_1}$,where $T_1$ is the temperature of the source and $T_2$ is the temperature of the sink.