Cross price elasticity
%change of Q of good 1/ % change in price of good 2 In the example above, the two goods, fuel and cars(consists of fuel consumption), are complements; that is, one is used with the other. In these cases the cross elasticity of demand will be negative, as shown by the decrease in demand for cars when the price of fuel increased. In the case of perfect complements, the cross elasticity of demand is negative infinity.Where the two goods are substitutes the cross elasticity of demand will be positive, so that as the price of one goes up the demand of the other will increase. For example, in response to an increase in the price of carbonated soft drinks, the demand for non-carbonated soft drinks will rise. In the case of perfect substitutes, the cross elasticity of demand is equal to infinity.