Changes

If Anthony sells only three widgets at $9, then that is four less than what a competitive market would sell. The social cost is the sum of (P-MC) over each of the withheld units, noting that the social cost for each withheld unit is different because the unit goes unsold at a different P.  

If Anthony sells only three widgets at $9, then that is four less than what a competitive market would sell. The social cost is the sum of (P-MC) over each of the withheld units, noting that the social cost for each withheld unit is different because the unit goes unsold at a different P.  

'''We assume that society would have purchased the unit at slightly less than the higher price, such $6 minus an infinitesimal amount.''' When the price went from $5 to $6, one unit went unsold and the loss to society was '''almost''' (P-MC=$6-$5=$1).  Likewise, another unit went unsold at $7 (P-MC=$2), another unit went unsold at $8 and another unit went unsold at $9. That total social cost is '''almost''' $1 + $2 + $3 + $4 = $10.  

We assume that society would have purchased the unit at slightly less than the higher price, such $6 minus an infinitesimal amount. When the price went from $5 to $6, one unit went unsold and the loss to society was '''almost''' (P-MC=$6-$5=$1).  Likewise, another unit went unsold at $7 (P-MC=$2), another unit went unsold at $8 and another unit went unsold at $9. That total social cost is '''almost''' $1 + $2 + $3 + $4 = $10.  

If instead Anthony sold 4 units at $8, then the social cost is '''almost''' $1 + $2 + $3 = $6.  The "almost" is so close to the number that we drop the "almost" and simply provide the number as the estimated social cost.'''

If instead Anthony sold 4 units at $8, then the social cost is '''almost''' $1 + $2 + $3 = $6.  The "almost" is so close to the number that we drop the "almost" and simply provide the number as the estimated social cost.'''