BTGmoderatorDC wrote:The restorative power of sleep is graphically approximated by the function −x^2 + 16x + 36, where the x-axis measures sleeping hours and the y-axis measures the restoration value. After how many hours does sleep no longer perform restorative duties according to the function?
A. −2
B. 4
C. 8
D. 13
E. 18
Multiplying the equation by -1, we get:
y = x² - 16x - 36
y = (x-18)(x+2)
The restorative power -- the value of y -- is equal to 0 when x=18 or x=-2.
Since the time cannot be a negative value, only x = 18 hours is viable.
The correct answer is
E.
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