Question 177198
c. Use the quadratic equation to determine the last day that tickets will be sold. (Note: Write your answer in terms of the number of days after ticket sales begin.)
Determine when sales = zero:
-0.2x^2+12x+11 = 0

x = [-12 +- sqrt(144 -4*-0.2*11)]/[2*-0.2]

x = [-12 +- sqrt(152]/[-0.4]

Positive solution:
x = [-12 - 12.36]/[-.4]

x = [-24.36]/-0.4] = 60
Sales are 0 on day 60
Last day tickets will be sold will be day 59
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P.S. I made a typo on the earlier solution.
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d. It's a peak because the quadratic opens down.
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e Peak occurs when x = -b/2a = -12/(2*-0.2) = -12/-0.4 = 30
The number of sales that day is:
s(30) = -0.2(30)^2 + 12(30) + 11 = 191
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Cheers,
Stan H.