what is the difference between negative and positive externalities
Externality Voluntary exchange is by definition mutually beneficial to both business parties involved, because the parties would not agree to undertake it if either thought it detrimental to their interests. However, a transaction can cause effects on third parties without their knowledge or consent. From the perspective of those affected, these effects may be negative (pollution from a nearby factory), or positive (honey bees kept for honey that also pollinate neighboring crops). Neoclassical welfare economics asserts that, under plausible conditions, the existence of externalities will result in outcomes that are not socially optimal. Those who suffer from external costs do so involuntarily, whereas those who enjoy external benefits do so at no cost.
Cross elasticity of demand In economics, the cross elasticity of demand or cross-price elasticity of demand measures the responsiveness of the quantity demanded for a good to a change in the price of another good, ceteris paribus. It is measured as the percentage change in quantity demanded for the first good that occurs in response to a percentage change in price of the second good. For example, if, in response to a 10% increase in the price of fuel, the demand for new cars that are fuel inefficient decreased by 20%, the cross elasticity of demand would be:cross body    − 20 %  10 %  = − 2  {\displaystyle {\frac {-20\%}{10\%}}=-2} . A negative cross elasticity denotes two products that are complements, while a positive cross elasticity denotes two substitute products. Assume products A and B are complements, meaning that an increase in the price for A accompanies a decrease in the quantity demanded for B. Therefore, if the price of product B decreases, the demand curve for product A shifts to the right reflecting an increase in A's demand, resulting in a negative value for the cross elasticity of demand. The exact opposite reasoning holds for substitutes.