I read the book volume 2. and I see the plot of demand and marginal cost. the book said “if market price were to rise. the firm’s demand and MR curve would simply shift upward”. But I see the Demand Curve imply the price is constant. how to understand the sentence??
It’s assuming an infinite number of sellers, each having 0% market share.
It’s a calculus thing.
but how to understand “the price were to rise” ?
Demand is perfectly (price) elastic: if the price rises by any amount, demand falls to zero; if the price falls by any amount, the demand becomes infinite. Therefore, the price cannot change; i.e., it is given and a supplier can take it or leave it.