r-JK-GT-14

unresolved Why are these functions called the “aggregate supply function” and the “aggregate demand function”?

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Let $Z$ be the aggregate supply price of the output from employing $N$ men, the relationship between $Z$ and $N$ being written $Z=\phi (N)$, which can be called the aggregate supply function. Similarly, let D be the proceeds which entrepreneurs expect to receive from the employment of $N$ men, the relationship between $D$ and $N$ being written $D=f(N)$, which can be called the aggregate demand function.