Understanding demand and supply curves

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Discussion Overview

The discussion revolves around understanding the demand and supply curves represented by the equations QD = 5600 – 8P and QS = 500 + 4P. Participants explore the graphical representation of these curves and the conventions used in economics for plotting them, particularly the relationship between price and quantity.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the graph of the demand and supply functions and questions why it does not follow the expected mathematical representation.
  • Another participant suggests rearranging the equations to clarify the relationship between price and quantity, noting that economics often treats quantity as a function of price.
  • A participant raises a concern about the inconsistency in the equations and the graph, questioning whether there is a convention for reflecting the supply curve along the line y=x.
  • One participant identifies a potential error in the supply equation, arguing that it implies an unrealistic scenario where supply exists at a price of zero, suggesting a correction to QS = 4P - 500.
  • Another participant acknowledges the unconventional nature of the graphing approach but finds it useful when integrating cost curves with demand curves.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the equations and their graphical representations. There is no consensus on the appropriate way to graph the supply curve or the validity of the equations provided.

Contextual Notes

Participants highlight potential errors in the equations and their implications for the graph, indicating that assumptions about the relationships between price and quantity may not hold universally. The discussion remains focused on the interpretation of the equations and their graphical representations.

dpa
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Hi everyone, I am having hardtime understanding this problem.
I have two functions:
QD = 5600 – 8P
QS = 500 + 4P

Why is the graph like the one attached and not the normal mathematical graph where supply curve starts from y=(0,-125)?
Do ignore the consumer surplus and producer surplus part. That is irrelevant.
How (and why) do we proceed in this way in our calculations?
 

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P is the dependent variable in the graph. Rearrange the two equations and it will make sense (or reflect the graph about y=x). This is the one really annoying thing about an introduction to economics, you just get used to it. They always consider quantity to be a function of price but interchange them when graphing.
 
Suppose I rearrange the equations,
then P=700-0.125QD [which is fine without reflection and same as QD=5600-QD]
But, next P=0.25QS-125 but the graph is for 0.25QS+125. Note the + and _ after QS. It works with reflection. but not normally. Is there a certain convention. I mean am I always supposed to reflect along y=x line for supply curve only?
But then suppose we had QS=-500+4P. Would its y-intercept be (0,-125)?
 
Sorry, I didn't notice that. Algebra is still algebra, somebody made an error. The supply curve graphed is not the equation given. My guess is that they intended QS=4P-500 because the equation as written makes no sense. It implies that a producer would be willing to offer a supply of 500 at a price of zero which is absurd. So your equation is wrong, the graph given does not match the equation given, and the graph given does not match the alternative equation that I provided. They clearly intended that the producer would not offer anything until the price hit 125 but they messed up the whole thing. You should point this out. You have not lost your mind. :)
 
alan2 said:
P is the dependent variable in the graph. Rearrange the two equations and it will make sense (or reflect the graph about y=x). This is the one really annoying thing about an introduction to economics, you just get used to it. They always consider quantity to be a function of price but interchange them when graphing.
Yeah, that is a little weird. But I find it's a useful convention when including the various cost curves (eg. marginal and average total cost) on the same graph as the demand curve, since cost is a function of quantity produced.
 

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