Chapter 4 Quadratic Functions and Factoring - 4.8 Use the Quadratic Formula and the Discriminant - 4.8 Exercises - Problem Solving - Page 298: 70d

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From exercise 70c, we know the x-coordinate of the vertex is $x=80$. Substitute this into the given equation: $y=-\frac{1}{640}(80)^2+\frac{1}{4}(80)+20\\=-10+40\\=30$ The maximum value is also the y-coordinate of the vertex, hence $y=30$. Hence, the maximum height the motorcyclist reaches is $30$ ft.