zexen_lowe on 13 December 2010
2nd one, since it's a downward facing parabole, you need to find its critical point, which in turn will be the maximum. To find that, you need to know when the 1st derivative equals zero.
Hence
R(x) = 90x - 0.5 sq x
R'(x) = 90 - x = 0 -> x = 90
So he has to manufacture 90 units, to obtain the revenue, you only need to replace in R(x) -> $4050
