Suppose you're given the following question:

Demand is Q = 110 - 4P. What is price (point) elasticity at $5?

We saw that we can calculate any elasticity by the formula:

Elasticity of Z with respect to Y = (dZ / dY)*(Y/Z)

In the case of price elasticity of demand, we are interested in the elasticity of quantity demand with respect to price. Thus we can use the following equation:

Price elasticity of demand: = (dQ / dP)*(P/Q)

In order to use this equation, we must have quantity alone on the left-hand side, and the right-hand side be some function of price. That is the case in our demand equation of Q = 110 - 4P. Thus we differentiate with respect to P and get:

So we substitute dQ/dP = -4 and Q = 110 - 4P into our price elasticity of demand equation:

Price elasticity of demand: = (dQ / dP)*(P/Q)

Price elasticity of demand: = (-4)*(P/(110-4P)

Price elasticity of demand: = -4P/(110-4P)

We're interested in finding what the price elasticity is at P = 5, so we substitute this into our price elasticity of demand equation:

Price elasticity of demand: = -4P/(110-4P)

Price elasticity of demand: = -20/90

Price elasticity of demand: = -2/9

Thus our price elasticity of demand is -2/9. Since it is less than 1 in absolute terms, we say that

Demand is Price Inelastic
**Next: Using Calculus To Calculate Income Elasticity of Demand.**

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