equation of a linear demand curve is D(p) = a-bp where (-)b is the slope of the demand curve.b= Δq/Δp. In meritnation's ncert solution (chapter 2, page-33,qn-23) this formula has been used.Then how can slope of a demand curve be Δp / Δq?Derive.

Shri Lakshmi,

Don't get confused between the slope of demand curve and price elasticity of demand. Slope of a demand curve measures the absolute change in commodity's price and quantity whereas price elasticity of demand measures the relative change in commodity's price and quantity. 

Slope of demand curve = $\frac{∆p}{∆q}$ and Price Elasticity of Demand = $\frac{∆Q}{∆P} \times \frac{P}{Q}$