What is demand curve? Or, Why demand curve is downward sloping? Or, Draw a demand curve from a demand equation. Or, Draw a demand curve from demand equation and calculate the slope of the demand curve.

 Demand curve:

A demand curve is a graphical or mathematical diagram that shows the relationship between the price and quantity of a product that consumers are willing to buy.

Analysis:

Let, imaginary demand equation, Qd = 10- 2p. In this equation, the different value of independent variable P gives different values of dependent variables. By presenting both values, we can make an imaginary demand schedule.

So, imaginary demand schedule:

Quantity      Demand                                (unit)	Price(Tk.)	Point
8	1	a
6	2	b
4	3	c

From imaginary demand schedule, price increases 1, 2 and 3, quantity demand decreases 8, 6 and 4. By this we get point a, b and c.

Drawing imaginary demand curve:

Putting the different prices from imaginary demand schedule in the demand curve, we get:

Curve analysis: 

The y-axis or vertical line represents “price” as the dependent variable, and the x- axis or horizontal line represents the “quantity demanded” as the independent variable.

From the demand curve we find that when price is Tk.1 demand is 8 units and the point is c. When price increases Tk. 2 and 3 demand will decrease to 6 and 4 unit and the points are b and a. By connecting the points, the demand curve is formed which is DD. Since price always has a negative effect on demand curve, it will have a downward slope.

Finally, demand curves are useful when testing and measuring the supply and demand of certain products within a competitive market.