The restorative power of sleep is graphically approximated by the func...
To determine when sleep no longer performs restorative duties according to the function −x2 + 16x + 36, we need to find the x-coordinate of the vertex.
The x-coordinate of the vertex of a quadratic function in the form ax2 + bx + c can be found using the formula:
x = -b / (2a)
In this case, the function is −x2 + 16x + 36, so a = -1, b = 16, and c = 36.
Using the formula, we have:
x = -16 / (2*(-1))
x = -16 / (-2)
x = 8
Therefore, the vertex of the function occurs at x = 8. However, we need to determine after how many hours sleep no longer performs restorative duties. Since the function is a downward-facing parabola, the restoration value decreases as x increases beyond the vertex.
As x increases beyond 8, the restoration value will continue to decrease. We can observe this by calculating the restoration value at x = 8 and x = 9:
At x = 8:
Restoration value = -82 + 16(8) + 36 = -64 + 128 + 36 = 100
At x = 9:
Restoration value = -92 + 16(9) + 36 = -81 + 144 + 36 = 99
As x increases further, the restoration value will continue to decrease. Therefore, after 8 hours of sleep, according to the function, sleep no longer performs restorative duties.
Hence, the correct answer is E: 18.