<p>Hi, I'm taking the AP microeconomics exam and I'm reviewing with the PR book. I have some problems about externalities graphs. This is what the book says:</p>
<p>Note that for the purpose of finding the social optimum it makes no difference whether the MEC is added to the MPC curve or subtracted from the MB curve. Either way the MEC creates the same sized wedge between MB and MPC and the resulting socially optimal quantity and price are the same. Likewise, a MEB could be subtracted from the MPC rather than being added to the private MB to yield identical results.</p>
<p>Please explain this to me. BTW, what does it mean to subtract or add to a curve?</p>
<p>Thanks for all the help in advance.
:)</p>