Boris K. answered 12/02/14
Tutor
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Math-Programming-Stat-Physics
Since the revenue R(P) = 85*P - 0.25*P^2 is a quadratic function its maximum is attained for the price P that makes the first derivative vanish, i.e., when
R'(P) = 85 - 0.5*P = 0
Hence, P = 170.