# 21 Model Building

$U_A = V - P_A - t |x-a| \\ U_B = V - P_B - t|b-x|$

where

• $$\bar{x}$$ = marginal consumer = indifferent consumer

• $$-t|x-a|$$ = Disutility of consuming a product away from ideal

• transportation cost parameters, weights on the disuility incur when they purchase a product different from the ideal point.
• V = reservation price = max max that the customer is willing to take

• x = location of marginal customer - customer who is indifference from buying from firm A and firm B.

To find the point that a consumer is indifference from buying from firm A and buying from firm B, we equate $$U_A = U_B$$

$V - P_A - t(\bar{x}-a) = V - P_B - t (b-\bar{x}) \\ P_B - P_A = 2t \bar{x}+ at - tb \\ \bar{x} = \frac{b-a}{2} + \frac{P_B - P_A}{2t}$

Market Share for firm A = $$\bar{x}$$

Market Share for firm B = $$1 - \bar{x}$$

$\pi_A = P_A (\frac{b-a}{2} + \frac{P_B - P_A}{2t}) \\ \pi_B = P_B (1 - \frac{b-a}{2} - \frac{P_B - P_A}{2t})$

$\frac{d\pi_A}{d P_A} = \frac{p \pi_B}{d P_B} = 0$

$\frac{P_B}{2t} - \frac{2 P_A}{2t} + \frac{b-a}{2} = 0$

and

$\frac{d^2\pi_A}{dP^2_A} = \frac{1}{t} < 0$

$1 - \frac{b-a}{2} - \frac{P_B}{t} + \frac{P_A}{2t} = 0 \\ 1/2 + \frac{b-a}{4} - \frac{3 P_A}{4t} = 0 \\ \frac{t(2 + b- a)}{3}= P_A \\ \frac{1}{2} - \frac{b-a}{4} + \frac{2 + b - a}{12} = \frac{P_B}{2t} \\ \frac{4/3} - \frac{b-a}{3} = \frac{P_B}{t} \\ \frac{t(4 - b+a)}{3} = P_B$