Profit = TR - TC
We can find maxima/minima by differentiating profit with respect to output and setting this equal to zero (and if we differentiate totals, we get marginals), so:
d(Profit)/d(output) = MR - MC = 0
So you get maxima/minima where MR = MC.
Then you can see from the diagram that if you are at Q1 and you increase output, because MR > MC, your revenue will increase by more than your cost, so profit increases. So Q1 must be a minimum point.
Alternatively, just imagine sitting at point Q1 and thinking about what happens if you increase / decrease output:
- if you increase output (as described above), profit increases
- if you decrease output, because MC > MR, your costs will fall by more than your revenues, so (again) profit increases.
So Q1 must be a minimum profit output level.