Explanation:

The slope of a line is a MEASURE of its steepness. It is given by the increase in the vertical coordinates divided by the increase in the horizontal coordinates. It simply indicates how MUCH the line rises per unit move to the right or how much it goes down as we move to the right.

The former (an upward rising curve) is said to have a positive slope while the latter (a downward sloping curve) has a negative slope. Thus, the slope of a demand curve is ∆P/∆Q. If the price falls we write -∆P/∆Q or if price rises demand falls, we write ∆P/∆Q. In either CASE, the slope becomes negative.

The slope of a curve refers to its steepness indicating the rate at which it moves UPWARDS or downwards. In the language of W. J. Baumol, “The slope of a line is a measure of steepness”. The slope of a demand curve shows the ratio between the two absolute changes in price and demand (both are variables).

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It can be expressed in the following way:

The slope of the Demand Curve (at a particular point)

= Absolute Change in Price/Absolute Change in Quantity

By applying this formula, it can be said that, when at the fall of price by Re. 1 (- 1) the quantity demanded increases by 10 units (+ 10), the slope of the curve at that STAGE will be -1/10. It is to be noted that in the case of demand function the price decreases while the quantity increases. So, the slope of a demand curve is normally negative.