Elasticity of Demand

E_{d}=\frac{\text { Percentage change in quantity demanded }}{\text { Percentage change in price }}

E_{d}=\frac{\frac{\Delta Q}{Q}}{\frac{\Delta P}{P}}=\frac{\Delta Q}{Q} \times \frac{P}{\Delta P}

\mathrm{TE}=\mathrm{P} \times \mathrm{Q}

E_{d}=\frac{\text { Lower part of the demand curve }}{\text { Upper part of the demand curve }}

E_{d}=\frac{\text { Lower part of the tangent line }}{\text { Upper part of the tangent line }}